Wow this atmosphere is INCREDIBLE!!! All the models look great too! The sound design is well done as well as the particles.
There could be slightly more story, maybe something about the creature and why the player is so motivated to find it (for example their parents looked for it and everyone thought they were crazy for it and they disappeared on one of their adventures. Now the player wants to avenge their parents).
One of the issues I found with the game is that you allow the player to move on before collecting all the orbs in the area. In the first area, there are 5 orbs but you only need 4 to continue. So I continued with the 4 orbs and was stuck with 7 in the next area. Then went back and got the 5th from the first area (my 8th) and finished the game. So make sure the player has all the orbs before continuing, either make the saw cost 5 orbs and the drill 8 or have a counter showing orbs in this checkpoint in the top right corner. Unless possible backtracking was intentional then it's fine.
Another very small thing I found was when the player first starts to move the submarine the sound plays immediately on press and sometimes like clicks? Maybe just make the sound quiet at first and then ramp up to fix this (the ramp-up would be similar to the propeller accelerating)? I'm very new to game development and have mainly focused on the coding side so I'm not sure if this would work but just an idea.
Clearly, the balancing of the difficult damaging objects and small corridors were checkpoints that restore your health. I think an additional thing you could add would be a hard mode where there are no checkpoints, but items that restore your health along the adventure (maybe wrenches scattered along the maze or just the orbs you collect also fully restore your health). If you die, you go all the way back to the start, but now there is a way to regain health along the way to make it still possible.
Overall this game is beautiful and polished extremely well. I could see it being ported to mobile as well with a simple joystick and buttons to activate the tools. With a few more modes added or some kind of replayability (Leaderboard with times? Randomly generated environment? Different maps? Multiplayer?) this could definitely be an even more incredible game :)