Wow, that was INTENSE! :O
On my first shot I managed to stay afloat but the boat never came back and the screen got black (which I felt as the sun going down and, with it, any realistic hope of rescue). I kept swiming and pressing the call for help button (although the screaming sound ceased) for a good 2-3 minutes after the screen was black, even though I honestly didn't believe anything would happen. Still, I kept trying and struggling for a while - I just wouldn't let go!
Then I felt it was the end. A quite sad one, in fact! Then I read through the rest of your description (the one flagged as spoiler) and tried again. I managed to see the intended ending - man, DESPAIR! Seeing the boat and being unable to call for help! It is horrible! The stamina simulation was great - I understand that turning the mouse wheel becomes less and less effective with time, right? Anyway, I slowly faded to black. It was quite depressive - which is probably why the game left such a strong mark!
Finally, I tried to get the "good" ending a few times, but once I let myself sink a bit in order to trigger the return of the boat, I can't seem to stay afloat anymore, no matter how hard I try (I nearly destroyed my mouse in the process! hahaha). I didn't manage to see the good ending, but trying to go for it and being unable too only reinforced the impression left by the intended ending. Man. I literally held my breath here. What the hell! Good job!
Overall, I'm rating this game really high, as it displays what I personally like the most in the strongest compo entries: a visceral experience that I will remember for some time, and that was built with few elements (all of which necessary, though).
Top rating in innovation, theme and mood! The theme is the very reason for the game to exist, and it is integrated in the very mechanics. I don't think it can get any better than that. Considering my genuine effort to not drown even on my first attempt (the one where I kept swimming and crying for help for like 3 minutes after having no audiovisual feedback) and then my successive failures at getting the good ending, I must say I was hooked. This is an indication of superb mood for me.
My only constructive criticism goes on the sound department. I really liked the music and sound implementation overall (the extra layer of instrument that gets muted underwater, specially, and the inability to call for help underwater which also contributes to the sense of despair), but I think the ambient music is quite relaxing, which kinda generates a cognitive dissonance for me. I think it would work better if, as stamina started decreasing and you spent more and more time underwater, the music changed to some heavy, fast percussions instead of something relaxing. Maybe some good old microphone recorded heavy breathing when above water would also increase the sense of despair, specially because it would be muted when underwater.
Anyway, all in all an stellar performance! I think that, unfortunately, a big part of this community will only favor traditional games that are fun in a traditional manner (the very bias you discussed in your post about consistent rating), so I'm guessing some people will just go like "ah, I didn't have fun, I'm actually feeling bad, so I'll give it 1 star", which might drag you down a bit in terms of score. Well, not for me. I wish that seeing bold entries like this was more common nowadays. Top ratings here, buddy, congratulations!