The art on the title card was great, would have loved to see the rest of the game in a similar style.
Music! This would have benefited hugely from some music! That is the one thing that truly sticks out to me.
The game felt a bit dull after having to redo the underwater part for the third time. There needs to be something the player can do other than just move during that part, some form of defense (or attack)!
Overall I think you could add some more cosmetics to the game, animations, livelier levels etc.. But I think there is a good game at the core.