Ok, so I've finished the game and I really liked it! Great job!
The second level is the worst part of the game. The monster is too hard, especially for the first enemy you face and the only one you can't fight against. You can find a spear in the first level right next to the key, yet it seems like it's useless? I think it would be better if whatever is chasing you there also had the "struggle mechanics" introduced later on. There's a lot of narrow corridors in the second level and once the monster catches you, you are basically done. The "struggle mechanic" really makes the game much better. I think most people who will try this game won't get past level 2, which is a huge shame.
The third level was a bit disappointing. It's very sparse compared to the other levels. Cool red color scheme aside, it's full of huge, empty rooms and hallways leading nowhere in particular. This level could use some more detail added and maybe also a "boss fight" with the torturer mentioned in the intro.
Regarding the third level, it's also not clear that what you are supposed to do is defeat the siren a bunch of times. I basically spent 20 minutes running around the place, trying to avoid her as best I could and in the end only got the bracelet by accident. Maybe have another note there that explains what you need to do, the only note I found basically just told you that the sapphire bracelet exists.
Fourth level was the one that needed a minimap the most. It was really easy to get lost there and you have limited air, so, really frustrating. I'd suggest having the bubbles that lead to a next area a different color, maybe golden? That should at least tell the player that they are on the right track. Also, the tentacle didn't actually damage you. The game says it did, but your HP stays the same afterwards, which seems to be a bug.
Aside from the previous criticism, I liked basically everything else. This is a really cool game and I'm looking forward to what else you plan to make in the future!