First off: love the sonar idea, I think it has a lot of potential. Was it inspired by barotrauma by any chance?
But I didn't beat the game because it was just frustratingly difficult for me.
- Deprussurization killing instantly feels really frustrating - it's a mistake that's realively easy to make as you try to use the sonar as little as possible and I think it would feel a lot better if you had, say, three hits until your sumbarine is destroyed by collisions - it would build tension as well, which for a horror game is good
- I just didn't get how you're supposed to avoid the monster. At first I thought it reacts to sound so you could possibly lure it somewhere with the sonar and then keep still while it moves past you to investigate that point - but it sees you as well, so there feel to be no way out of long dead ends(of which there are seemingly many). I guess it moves to investigate the sounds of sonar from any point? And I also noticed that it makes some sounds and I could determine where it is thanks to spatial audio(good job on that part) but as I can't really hide from it and must constantly use sonar to find a way forward while avoiding collisions that information just wasn't helpful to me.
I would like to end this with saying that the interface is awesome and sets the vibe really well. Best of luck on your gamedev journey and I hope my feedback helps!