Very pretty art! Music is appealing as well. Unfortunately, the game itself needs a lot of work, mainly under the category of polish. Here are a few problem spots:

• Movement in general doesn't feel great. This is partly due to the fact that the walking animation is just way too "slow" for the movement speed. Falling also feel too "floaty." It's true this is an underwater game, but just a light touch of downward acceleration would help sell your character's weight.
• The bubble powerup feels sluggish to use, since you either have to wait until the bubble pops to move again, or you have to drop the shell (which means you'll need to be somewhere where you can drop the shell safely). Being able to press the powerup button while on a bubble to jump off would be really cool!
• Your game has a pretty high risk for soft locking. Any of the platforming sections or the sections with the fish pushing you back could easily result in the player soft locking the game if they accidentally drop their shell in the wrong spot. 
• The switches don't seem like weighted switches! It's hard to tell they're being "pressed". This might be because of the way the animation plays.
• Fish puzzles can be broken by just going through them with the bubble powerup. 

Jam games are always tough - I imagine some of these things came up and couldn't be fixed, but it's always tough to tell "didn't have time to fix" from "didn't think to fix." Good work on finishing a game!