I noticed that there were not any H20:just add water and Hawaii five-0 crossovers, so I decided to make one, I hope you like it. Please not it is without a beta and I am only 14. Note this does contain an OC, Arianna, I could change it if people didn't like it, please tell me what you thought.

Diclaimer: I do not own neither Hawaii five-0 (2010 series) or H20: just add water.

All I wanted to do today was have an amazingly hot bath, or a swim, no a bath. I needed to just relax, let the water work its magic on my stressed muscles, it had been a long week after all. My name is Arianna and I am a mermaid, I was changed a long time ago in my home of Hawaii. My mum had to move to Australia for a navy post, she was always on the ship and never had any time for me, so I was shocked when she took a week off just to spend with me and my annoying brat of a younger sister. Here I met with four other mermaids and we became best friends, their names are Cleo, Bella, Emma and Rikki, it was strange to me to finally have people to share my secret with and that actually understood what I was going through.

I hurried through the door and went to the bathroom, shouting a quick 'hey' to my annoying mother, I quickly closed the door and began filling the bathtub with water. Once it was ¾ full I pulled my clothes off and plonked myself into the bathtub, looking over strangely as I turned into water for a split second before my legs started to fuse together, in my opinion being a mermaid is just the best. I silently laid there just remembering all the happy memories that had occurred from being a mermaid when I heard the door knob turn. I panicked and started to twist and turn, hoping that I could cover myself somehow, but it was too late, my mum walked in and gasped as she saw my brilliant now-purple tail. It took about five minutes of awkward silence for me whilst she just gaped at my tail before she said something.

"Dry yourself off young lady, your father and I need to talk to you." And with that she turned and left yelling my father's name as she went down the corridor.

My stomach flipped, we kept the secret for a reason, others just wouldn't understand and that's what category my mum was in, I could see it on her face; disgust. With a heavy sigh I pulled the plug to allow the water to drain from the tub and dried myself off with a towel. I could hear the urgent fighting of my parents as they discussed what to do with me, I caught the words Dr. Denman and knew I was in for it. I could connect telepathically with other mermaids, we all could after some training that one of the local Spirit Gods in Hawaii taught me and what I passed onto my friends.

"Guys, guys." I thought urgently focusing on my breathing and feeling the magic within myself.

"Yeah, what is it Arianna?" Cleo answered, I took a deep breathe, I needed to focus and not get worked up or I would be able to tell them in time.

"My mum walked in on me in the bath with a tail. Now she and my dad are talking about giving me to Dr. Denman. I'm scared, I don't know what to do." I thought in an urgent voice and I could feel that Bella and Emma had joined in the conversation.

"What, she wouldn't do that." Bella said urgently, off all our mermaid friends, I got along best with Bella and Cleo.

"My dad is literally calling her now." I said in a panicked voice.

"Ok, at the first chance you get; escape, go to the cliff. We'll all be there waiting for you, just relax, if you feel you need to, use your powers. We have to tell Rikki. Good luck Ari, it looks like our secret is getting out today." Emma said in her bossy voice, but I was glad for her bossiness now, it was what I needed. We all agreed and went off to do our separate things. I sighed as I looked down and saw that I had no tail, I got up and went into my room to change, I shoved my wallet along with my passport into my jeans pocket so I could leave to any country. I gulped and walked out into the living room where an angry mum and dad were waiting for me.

"Sit down Arianna." My dad said, his face expressionless, although I could tell he was angry by his use of my full name. I sat down nervously and started fidgeting with my hands.

"Well, you know why you're here." My mum said, her face was red with anger. I didn't understand the anger, it was strange to me.

"Because I'm a mermaid?" I asked in a monotone, my parents looked furious at me.

"Mermaids do not exist, what you are is something weird, so we called a marine biologist Dr. Denman about you." My father said, his face going redder and redder.

"Oh yay, the mythological creature psychopath." I said sarcastically, "Hope she doesn't dissect me." My father stood up, he looked as though he was going to hit me but I was ready for him, what I wasn't ready for, was my mother. She slapped me across the face so hard, I lifted a tentative hand to my face and it felt red-hot and I felt tears gathering at my eyes.

"You will not talk about her like that, Dr. Denman is a good friend of mine." My mum said, raising her had to strike again, I was ready for her this time, she went to launch at me again and I blew my mother and my father into the wall and smiled as I heard a loud thump. I knew I needed to act fast before they knew what was happening. I grabbed the keys to my dad's car and ran out the door quickly to the car and threw myself in the driver's side and sped off to the cliff. Tears rushed down my face as I was driving, going way too much over the speed limit.

I finally reached the cliff but was disappointed to see that none of my friends were there yet, I went over to sit at the bench, letting the tears flow freely down my face as I waited for them. With my in-tune fish hearing I could hear people approaching me but I knew it wasn't my friends, it was my family they had known where I would go. They sounded like they were about 250metres back so I ignored them for now, but I was ready for everything. My hand twitched and the wind started to pick up, Dr. Denman didn't suspect anything of it because she hadn't experienced the full wrath of our powers.

"We just have to ensure that it doesn't go in the water or we'll never catch it." Said a voice that I didn't recognise. Suddenly I could hear more people, other families. My mum obviously being the gossiper that she is probably told everyone what I was. I quickly got up of the bench and slowly walked over to the cliff-face preparing myself to jump.

"It's ok Ari, we made it. We're right behind you. We won't let anything happen to you." Said the sweet voice of Bella, I smiled and knew she would understand.

"Arianna Rose Fletcher, you stop right there." Said the angry voice of my father, I rolled my eyes, but stopped and turned to face him with a malicious glint in my eye.

"Right, now Arianna, I simply need you to come with me so I can run some tests." Said the voice from Dr. Denman, in a calm and slow way as if I was mentally slow, I scoffed and turned back to the ocean and took another couple of steps forward.

"Arianna, what do you think is going to happen after you jump into the ocean, where will you go? You have no money or anything to help you out there! Don't be stupid and go with the doctor." Said the stern voice of my mother, I was enjoying this a little too much, these people were meant to love me unconditionally, but of course, in their eyes I had to be fully human. I turned to face them.

"Do you like tornadoes?" I asked in a mysterious voice.

"Stupid girl, it's clear that whatever drug she has taken has gone straight to her head." Said Mrs. Gilbert, I could see the hurt in Emma's eyes for split second before it became determination as she came over to stand next to me.

"Why do you think I quit swimming lessons mum, all the clues have been right there. Doesn't everyone just love blizzards?" Emma asked glaring at her mother.

"Bella, these new friends of yours are strange, I do hope that you are not taking any of the drugs that they have been clearing doing." Bella's eyes were murderous as she came over to stand next to my other side.

"We are not stupid or drug addicts, we are something that none of you can comprehend. I do hope you guys like tsunamis." Bella said, catching onto our game.

We heard Cleo's father scoff and her little sister roll her eyes before Cleo decided to join us, her father grabbed her arm and tried pulling her back but she pulled herself free and joined us. "I am not scared of the ocean like everyone thinks, I love it but I love hurricanes more." Cleo said with a smirk before Rikki waltzed in a joined us.

"I personally love fire and lightening." She said laughing at the expressions on everyone's faces. Dr. Denman stepped forward.

"Come on guys, think this through, you can't just through water at us and try to escape, its not possible, one of us will get you." She said with an evil smile on her face.

"Rikki get the cameras and everything, Ari, do you think you can sing whilst making a tornado?" Bella said, stepping back slightly.

"Where should we go?" Cleo asked, fear present in her voice.

"Hawaii? It's where I'm from and it's a beautiful place." I said with determination.

Rikki stepped forward and made use of her magnetic powers and destroyed all of the cameras that were flashing magnificently in our direction. I stepped forward and started to sing, just the song that comes to mermaids when they have this power, no words, just notes. It only took a few seconds before everyone started to get very sleepy, they all started to collapse under the weight of exhaustion, everyone except Dr. Denman, she smiled as she saw what I was doing didn't affect her, yet anyway, we had a special show saved, just for her.

As I sung I lifted my hand up into the air and made a loose claw pointing straight at Dr. Denman, the other four girls did the same with their powers and all of a sudden a magnificent weather show was playing out in front of us. Rikki had thunder and lightning and fire all around Dr. Denman, Bella had ginormous waves crashing around her as the ground almost seemed to rattle, Cleo had a large hurricane around her whilst Emma had a blizzard raining down on the evil doctor. She covered her face and crouched down to the ground, something about that image brought a smile to my face, we brought the intense and unheard of storm closer and closer to her, over the roaring of the wind we heard her whimper a little and I instantly felt bad. I ceased the storm and so did the others, but I was still singing.

"Wow, girls, you certainly have gotten more powerful, but I still think you're stupid for leaving, somehow we will find you." Dr. Denman said, the eil glint in her eyes returning.

"Do you know what Ari is doing right now?" Rikki asked innocently.

"I have no idea why she feels the need to sing so badly." The doctor said sarcastically.

"She's wiping everyone's memories, no one will remember this day, not even you." Emma said, a smile spreading across her face, as fear spread across Dr. Denman's and she tried to turn and run. Rikki had beaten her to it, there was no way past from the massive inferno of extremely hot fire. She slowly turned to face us again.

"We would never hurt you, but you seem to want to hurt us. We just want to live in peace, without nosy scientists poking their noses into everything." Cleo said, her voice fierce, they all turned to look at me as I turned to look at Dr. Denman, looking her right in the eyes. In the last couple of years, I have learnt to harness this power, I can make large gaps disappear from their memories, I can also take away a memory from years ago.

I sung louder, my voice echoing off the ocean as the creatures were helping us, I quickly scanned her memories whilst she was under my trance and found the one I was looking for; Emma, Rikki and Cleo in the moon pool as mermaids with scared expressions on their faces and anything since then that had to do with mermaid, I removed it all. She would have large gaps of memory, but it was for the best. Once I was finished, I stopped singing and nodded to the girls, wiping that many people's memories was a tough job.

We all turned away from the carnage that we had created and started walking slowly towards the cliff-face. I looked down at the ocean, it was like it was calling to me, all of us, to join it. We looked at each other for barely a moment as we linked hands and jumped off the cliff together, like we always did. The water was warm and beautiful as always, we wasted no time in heading straight for our beautiful mermaid home, the moon pool at Mako Island. We were saying our goodbyes to our home. We surfaced at the moon pool and looked it over sadly.

"You know what." I said, determination clear in my voice. "We should block the entrances, all of them, the volcano too."

"How are we going to do that?" Emma said with annoyance.

"Easy, Cleo makes the water fit and Bella hardens it, nothing could get through that." I said, looking at Bella who seemed to be agreeing with me.

"Ari's right guys, we can't leave this place, for who knows how long to leave it at the hands of some other person who wants to harm it. I couldn't see that again." Bella said, looking at Cleo and Rikki, Emma seemed to have warmed up once she realised our determination behind it. Cleo nodded and looked at the moon pool in determination, her arm in the air as a tentacle of water came out, she took it over to the hidden entrance at the top and Bella followed her. Cleo made it fit perfectly, and Bella turned it to jelly and then quickly hardened it, she kept her hand up to harden it for almost a minute, to make it harder than rock. They did the same for the volcano top and made one for the mermaid entrance, but we had to get out first.

We looked at our home sadly and turned to leave, but as we went to go back into the pool, a tentacle of water shot out at us and one of the rock pools dried up, leaving behind places for five crystals. We looked at each other and knew what we had to do, we ripped the necklaces off our necks and undid the biding holding the crystal to the string and placed the crystal in the grooves of the rock. As the last crystal was placed there, the tentacle started to move again, it went around all of us and pulled us into the moon pool, as something like a video started to play. It showed us, now, watching this video and then leaving to go to Hawaii, I could tell from the pictures. We felt like home, we got a house for the five of us and went to school. It showed us some of the things we had done and the things we were going to do and finally it showed us coming back here, we looked exactly the same, but it told us that it was the year 2045. And then it ended, and I have to admit, I did shed a tear or two.

"What was that?" Emma said in awe.

"That was the moon pool saying goodbye to us." I said, still sniffling as Cleo put her arm around me.

"So, we're coming back in in 2045, but we look exactly the same, are we immortal?" Rikki asked, half nervously, half excitedly.

Bella nodded, her eyes fixed where the video was just playing. "But what about Ms. Chatham, she was really old." Said Rikki, earning a look from Emma.

"She gave it up." I said simply, the rest looked at me like it all made sense now. Rikki pulled herself out of the pool and quickly dried herself off and walked over to where our crystals were.

"That's right, she may have been immortal once she was a mermaid, but she was only one for a short time, once she gave it up, she started to age again." Cleo said excitedly, Rikki returned with all of our crystals and string and handed them to each of us and we quickly restrung them and hung them around our necks.

Emma sighed, "We should get going, Hawaii is a long swim." We all looked at the magical moon pool and each thought how much we would miss it.

"We'll never forget you and all you did for us, thank you." Said Cleo, under normal circumstances this would have earned a nasty comment from Rikki, but she was thinking the same thing.

"Just think girls, 32 years and we'll be back. I already can't wait." Rikki said, her eyes gleaming with tears. I nodded solemnly and we all started to leave. After the last one of us had left Cleo lifted the third tunnel block and lifted it into place and stuck it securely. With one last final glance at each other we swam away towards our new home and our new life.