Top-notch mood.
I liked the visuals, the alien Cthulhu-esque looks of the body parts was very cool. Perhaps a bit more effects when we hit the parts to feel a bit more gross, instead of just disappearing. Just to match the descriptions the narrator give us. I just thought the playable screen on Itch was a bit too large, lost a bit of screen space until I managed to find the full screen button.
Music was brilliant. The moment I saw Moustronaut was involved I knew it would be amazing. Sound effects are also very fitting for the claustrophobic gross environment. The narrator is on point pretty much all the time. I'd just make a few lines when the player picks items to be less cheerful, more sullen.
The gameplay was pretty simple to understand. Some crevices are more challenging but it never feels frustrating or rage-inducing. Sometimes I wouldn't pick the oxygen tank because it collided visually but it's collider seemed to be a bit more at its top. Also regarding collisions, perhaps there could be a grace period or a cooldown for the submarine hitbox, because when you get stuck it's easy to hit walls multiple times and lose a lot of life. One thing that felt unfair though was the submarine slowly sinking while I was reading the logs, there was one time in which it hit the wall while the narrator spoke and it registered as damage.
I thought the upgrades were a neat addition, although it didn't feel very fitting to have the player being forced to die to upgrade the ship. At the end I got to the heart but I wasn't heavy enough, I shouldn't have to kill myself in order to advance.
Overall, a super cool and moody entry, congratulations!