Hiiii, I worked on the audio and i'd love to show how I did it, what part of it do you want to know more aboutt?
In general though, slowing any audio down a bunch made really nice bassy drones for underwater tones, using reaally long reverbs (Used Valhalla Supermassive for that, it's free!) helps for atmosphere too
as for sound design, again pitching things down/up. I had to overlap lots of sounds made using my mouth that I thought would sound good, (drinking water, pronouncing weird syllables, even farting noises were extremely useful hehe) let me know If you want to know more!! I'd be glad to talk more about it!