Based on a prompt I saw on Tumblr: "4x16. As the water rises to her chin, she whispers that she loves him."

Her seatbelt was stuck. Her car had just been pushed into the Hudson River, and they were rapidly sinking, and she couldn't get her belt off. Damn.

"I'm stuck," Kate said to Castle, trying to keep the panic from showing in her voice. Already, both of their car doors were jammed shut, and the crash had knocked her gun out of its holster. Now her belt was stuck, and if she couldn't get it undone…

"What do you mean, you're stuck?" Castle asked. God, was he stupid? She really had to elaborate on what "I'm stuck" meant?

"I mean, my belt is jammed," she said, a hint of annoyance showing in her voice. "And the seat won't move."

Castle immediately reached over and began tugging at her belt, trying to free her, but it was to no avail. Kate Beckett really was stuck in her seat, unable to move, as she and Castle slowly plummeted to the depths of the river.

"Okay, what about a knife?" Castle finally asked. "Do you have a knife?"

A glimmer of hope made its way into Kate's heart. She had a knife. "Yeah!" she answered quickly. Her heart sank as she suddenly remembered where she'd stored the knife, just this morning. "… in the trunk," she added sorrowfully, the glimmer of hope gone.

The water was filling the car rapidly, and Kate knew that she and Castle were running out of time. If they couldn't find her gun, they were as good as dead. Their only hope was to shoot out one of the windows, blast her seatbelt off, and escape out the shattered window. Castle began feeling around his seat for the gun, his hands surely freezing in the cold, murky water.

After what seemed like hours, Castle turned to Kate, a look of concern in his eyes. "I can't feel it."

He couldn't find it. This was the end. She'd never be able to tell him how she felt. Unless…

"Castle…" she started, not trying to disguise the panic in her voice anymore.

"Okay, hang on. Hang on," Castle said reassuringly, interrupting her before she could finish. He was so convinced that he could save them; that they would live. Didn't he know that not every story finished as neatly as one of his books? Couldn't he see that this was the end; that they were going to die, together, in her submerged cruiser? He was a fool. An optimistic fool.

He grabbed her flashlight, switching it on. "Just stay here," he said, diving under the water before Kate could protest. Obviously, she had no choice but to stay where she was; she was stuck. He hadn't meant it literally, though. He'd been telling her to hold on; to not lose hope; to not give up. He was still determined to save her life.

So she held on, trying her hardest to believe that Castle would find her gun and save them. He popped up for air a moment later, excitement in his voice as he said he'd found it. But it wasn't in his hand, and he dove down again.

Maybe there was hope after all. Maybe there really would be life after this crash.

But Castle didn't come up. He had been under the water for a long time, and there was no sign of him or her gun. The water was almost covering her, and she was panicked, calling out his name. She reached for him desperately, her hand swimming under the water. If they were going to die, she needed to be touching him; holding on to him. But she couldn't find him. She angrily jabbed her elbow against her window, repeatedly, but it was of no use.

She was dying—they were dying—and he'd never know how she felt. She'd been too afraid to tell him, that after all this time, after everything they'd been through together…

The water was to her chin. Kate lifted her head high, taking one last desperate, choking breath. "I love you, Castle," she whispered. "I love you."

Tears filled her eyes, but were quickly washed away by the water that now rose above her head. She placed her hands on the steering wheel of her car, in one last attempt for control, and closed her eyes as she felt herself drowning.

This was the end. And she'd waited too late.