After searching a bit about underwater-themed games I came across the ikachan game developed by Pixel when I was developing the engine that I would later use for Cave Story, it's a short but nice game where its main distinction is with other 2d exploration adventures (Metroidvanias ) is the fact that mobility is governed by impulses instead of a free movement more similar to floating that other games use or being tied to the ground and considerable gravity as in characters that are represented on solid ground. It's a pretty well done game that I understand after porting to the Nintendo 3ds with various additions, but its original version is free to play on the Cave Story page, along with other Pixel works.