Soothing atmosphere due to the music and the slow movement. The game is brutally difficult, but the controls are tuned just right. It really gives that weighty underwater feeling without sacrificing any precision. The red blinking is good user feedback.
The farthest I got was the plastic bottle graveyard, although most of my runs ended much earlier than that. It certainly kept me trying for a good 30 minutes, but eventually I had to throw in the towel.
Fun game, nicely done!