["Well we were," Zeph pouted, "'till you had to go and ruin the fun, Mom."]

You seem to be getting dialogue mostly right but stuff like "pouted" isn't a speech verb. Punctuation/capitalization treat it like the start of a separate sentence. ["Well we were." Zeph pouted.]

[his Mom ]

Not capitalized.

[Zak was nowhere to be seen. Zeph had run, the wave drawing ever closer, his brother following along as fast as he could...
But Zeph had always run just a little faster.]

You do a good job of repetition and echos throughout this, and I really like how this one turned out.