First, points for originality, playing as a fish going up a river, pretty nice.
Music choice really good, quite fitting for the game.
A bit too much red though, makes it hard to tell things apart, could use some more color variety IMHO.
Also although the core gameplay was nice, it also felt like it lacked variety after multiple levels. Something like needing to activate triggers, moving obstacles, upgrades, etc. Didn't manage to make it to the end so maybe there's more variety later on? Still some more variety earlier would feel nicer.
Something that annoyed me it's that sometimes you can swim right next to the coast and other times you cannot, making it harder to plan a route when you don't known if you can enter the tiles near the coast or not.
All around had some fun, no big bugs, nice work!