I thought the character was very convincing. I pictured the thing at the bottom of the lake as being something like an angler fish evolved to capture humans--using that bioluminescent glint to lure people to the bottom and then reaching up with fingerlike tentacles to try to keep you down there until you drown.
The story felt a little incomplete to me, like it didn't all wrap together, I'm not exactly sure why. Maybe because the glinting possible-beastie felt a little removed from the rest of the events. I can't quite put my finger on it, thematically I think it makes sense but it felt kind of incomplete somehow. Not super useful feedback, I realize. But I did find the characters very convincing.
And I have had a near miss with diving/drowning. I am not a strong swimmer, and I never have been. I never really got the knack for it when I was a kid though I've gotten to a point now where I can backfloat for quite a while and can doggy paddle and do some very basic strokes. When i was in high school I was at the lake with some friends, and we were kind of hanging around a buoy that marked the end of the swimming area. People were diving and seeing how far down they could go and I got it into my fool head to follow the chain of the buoy and see if I could reach the bottom. I don't know how deep it was at that particular point, but even with the aid of the chain to follow down it was further than I could make it down, and I pushed a little bit further than was safe for me, a fact which I only really realized when I was about a quarter of the way back up and was starting to feel pretty desperate to take a breath. I swear that chain was at least twice as long going up as it was on the way down and I think I was only a hair's breadth away from gasping in water. Maybe the people I was with would've saved my ass, but I wouldn't count on it, so my life could very easily have ended there. I did not make that mistake again. That moment did come to mind as he was being held under the water.