The 2.5D angle of this game almost reminds me of old 32-bit games. Interesting! To improve the platforming, I would suggest making the walls interact with the player's momentum less, having more precise landings or bigger floors (this would also justify somewhat slower acceleration) to jump onto for coin sections like the beginning's, and slightly slowing down the swimming speed.
That aside, the perspective shift was really neat and it definitely feels like a game that needed to use 3D models rather than something that was merely experimental. I can tell a lot of thought went into this. You had a creative solution to making the water area without relying on translucency effects. Kinda started laughing at the game over screen too.