The game has a lot of potential, but barely has any content at the moment, you can get the treasure, and maybe the dolphin, but other than that, there's really no fun to be had.
Currently you don't really feel like you are under water, so maybe you should add a blue tint over the screen and a wave shader, to make it better.
Also a thing I say to literally every Unity game that doens't have this POST PROCESSING, just some little bloom and vignette will go a long way to make your game look so much better, so please, do that.
You managed to keep the pixel size consistent in the game game really well, but the UI has a different pixel size, so you should fix that to make that look better.
And then the game needs some swimming and text appearing sounds, text that appears slowly feels so much better if it has some sounds.
And then a transition, because the hard cut from the menu to game doesn't look that good.
And then some attacking feedback, like the current fish attacked sound is way too unsatisfying, it's so quiet, so you feel really weak and also a fish died sound would be good, to for it to feel even better. Then attacking and fish dead particles, because those also make the attacking feel better.
And when you run out of oxygen and supposedly go back to the surface, you should make it so the player starts automatically going up and then the screen fades to black and then away when back up, because it just cutting doesn't really look that good.
And then a really small nitpick for the last thing, the wall at the ends of the maps are really cool, but there are 2 layers of them, and it would look so much better if there were some parallax.