This is some really good feedback, my friend. Thank you so much.!
The boss battle is long unless you use the electric tail. But I've been on the fence about increasing the damage dealt overall and I think you've convinced me, so I'll shorten that boss fight a smidge.
I agree that when I start swimming really fast I can't see any enemies ahead of me. I think I want to fix the camera by zooming it out just a little.
I think conveyance is so important in games and you're right about Big Fish feeling too open near the beginning. I really wanted the player to not feel trapped in a tiny game for the jam. I intend to use an accordion style of progression, keeping things linear near the beginning and end, while slowly opening the game up to the player throughout the middle, but for the jam period, I didn't unfold the areas as well as I could have. For the SuperMVM, however, I plan to have a much, much larger game with more bosses and powers. That will let me build a better flow for the map layout.
Thank you again for your insight! I've been so happy with how great of a community this is and I hope to see you and everyone else again at the SuperMVM!