A girl dissolves, and reforms, for her own reasons.
This water is so much warmer than the water she was in just a second ago!! Which is good because that was starting to get to her. And it's so bright in here!!!! She can see all of her new body even though her eyes are so much worse for seeing in the dark. She leans against the wall of the pool (with its straight straight smooth surface!!) and tries to pick up a foot to get a closer look at it. She falls over.
Her feet are very cool but even knowing humans need to breathe air she finds herself needing it sooner than she expected. She didn't have to dip back under the water that often as a mermaid. It's fine, she can swim to the surface, and gasp, and - okay, she's not really sure how to stand back up - this smooth smooth wall is very slippery and she can't grab onto it well enough to support her weight -
Okay yeah looks like it's time to go lend a helping hand. Won't do either of them any good if he falls in, though, so he walks around the edge of the water rather than running. He lies, belly-down, on the floor, and grabs at her arms until his grip is solid enough to set her on her feet.
Here on this cuboid is the animated image of a fish falling above the surface of the water and then diving down into it. Ariel taps the screen a bunch of times on one side until it displays a redheaded girl with a fish tail, then taps the center twice and the side once again. He displays the paused video to everyone else (briefly flinching away from the sight of Cyllene but relaxing once he sees she's dressed).