Thanks, yeah you gotta go back to the first area after getting the shoes if you want to proceed.
The acceleration is a bit slow because I wanted it to feel like you're underwater. First I also put a variable jump height in the game but it didn't work well with the boots upgrade, I could have assigned the boots to a different button though. The enemies do show it when they're about to shoot projectiles (except the wall shooters) but it can be hard to see, for example, the pufferfish just opens its mouth.
Really wished I could have implemented a map system to make the game easier to navigate as it is quite big.
Thanks again!