Does Uma have water manipulation powers in the movies or book? Or is that a fanon thing?
Short answer: …yes?
Long answer: Uma’s shown as having a bunch of different powers across the movies, uts, and novels, but the only moments I can think of as actually showing her being able to manipulate water are her transformation scenes: when she goes from human to... caecalia? idk whatever she is, and vice versa (once at the end of d2, then again in her first scene in d3), and some of the stuff she does in Under the Sea, if you want to take all that stuff as canon.
However, since the writers didn’t bother to come up with any kind of consistent magic system for the descendants universe, you can take that to mean literally whatever you want. Is the water that rises and swirls around her just part of the transformation, or is it specifically water manipulation? Who knows? Certainly not the writers!
Some other abilities Uma’s been shown to have in canon:
In the novels:
Shapeshifting - she becomes a tiny squid as well as a giant caecalia.
Weather manipuation - she creates a massive storm (I’ve seen some people say this is hinted at in d2 as well, during it’s going down - the sky gets darker and more storm-like and ominous during the number)
Telekinesis? Idk what to call it but she calls the trident towards her and it moves when she does.
In Under the Sea:
Manifesting herself... in water?? As water? A water version of her? She can do the water clone jutsu or smth I have no idea
Manifesting herself and/or an image of herself in bubbles and reflections
Controlling sea life
In the movies:
Spell casting - she can clearly use both a love spell in d2 and use spells with Mal in d3
Shapeshifting, again.
Additionally, she reignites the ember, which... I have no idea what that says about her power. Could anyone with magic have done that? Why can she reignite the ember if only someone related to Hades can use it? It’s meant to be immensely powerful, right? How powerful is Uma?
Also, to find some of this I went back and rewatched under the sea, and paused at a truly spectacular moment:
You’re welcome.