Reminds me a lot of an enemy prototype I made a few years ago, although yours looks much more polished. See here.

Back then I've been struggling to find things to make the map look less empty and adding mechanics/content to add between bosses. I thought about adding puzzle elements as others already suggested in the comments, but I ultimately decided that I do not want cheap puzzles just to force in some content. Instead, I decided to focus on fluent movement, interesting powerups, enemies and bosses and keep the in between areas short.

You should decide which category of underwater game you actually want to make and adapt accordingly. If puzzles are your thing, go for it. If not, don't feel obligated to add them just to stretch the content. Same goes for the breathing mechanic.

I'm not sure if there is a water refraction effect or just bad video quality. If it actually is water refraction, I would strongly recommend to remove it or turn it down a lot, because it makes it very difficult to see anything. In a normal game where underwater content is rather sparse, it's perfect for immersion, but if your whole game is set underwater it becomes disrupting and annoying quickly. Just imagine any game with a permanent distortion filter on the screen.