Crabs? Bats? Swimming? Hmmm, that reminds me of something...
The overall execution is incredible! There are so many nice little touches: surface-based footstep sounds, separate animation for crouch, swimming transformation, water shader... You even implemented fully functional slopes!!
That water body collision is truly a pain to handle. In our game I ended up switching the collider between a rectangle on land and a circle in water, but it still feels a bit weird.
It's a bit of a shame you teased with different routes that require a double jump, and then cut that ability out of the game ¦3 (but that's better than creeping the scope, well done on prioritizing!)
The only problem I had was a very big input lag. Everything happens with a noticeable delay.
That's a great foundation to expand from, well done!