Nice, solid base for a game and not bad for a beginner. Here is some advice:
- Do not include copyrighted music from other games in your released games! This might actually get you in trouble (especially when you sell such a game). I also do that sometimes, but I remove the music before sharing my game online.
- Try not to mix assets from various different artstyles into one game. It would be best of course to create your own assets, but sticking to one style / type of asset also helps a great deal.
- The underwater movement is a bit weird, I think simple WASD movement similar to the overworld (with some friction) would have been better
and many more small things that would break the scope of this comment.
Don't let this discourage you, everyone starts somewhere ;)