This is a fun game. On the first level, the side currents can lead you out of the border, which feels to be the safest route down. Also replaying the level forces you to lose your bubbles to exit the level I believe. I liked the details of the terrain and the fish. Those corals look to be Mario inspired :)
I think there's not enough mermaid games! Buttons A and B just slowed me down, so maybe you can do something useful with them instead. Avoiding enemies and traveling the vast ocean is quite relaxing.
The source code is very nicely organized, I think you probably have a lot of experience in game dev, although in my opinion assets shouldn't be under the sources directory. Looking at them, is there a pineapple somewhere? I couldn't find him!
If there was music or sfx, I missed it sadly. I played this game on a virtual machine with no sound on.