Interesting way to use the theme!
Even if there isn't much gameplay yet, this way it's still easier to switch direction of where you want to take the game. A swimming simulator where you just discover places underwater (caves with air pockets and some cool secrets?) could be fun? With some relaxing ambient music and background sounds it could be an immersive experience to relax after school/work. Also, using up HP instead of dying immediately after running out of air is a nice touch.
By coincidence I've made a similar type of game for a jam before, so it's interesting to see how others approach the idea differently.
If you're curious: https://djentalist.itch.io/peaceful-diving