Really great stuff! I was impressed by how fun the game was to play; the movement was spot-on.
The addition of the game getting darker as you get deeper was a well-needed touch to make the game more interesting and difficult. The leaderboard was a good touch to keep me playing for more than one round as well.
One thing I would add is some sort of feedback to let the player know that they are taking damage. For instance, I couldn't quite tell if the fish were close enough to hit me or not, or if the rocks did any damage at all.
Overall, I think you did a great job making a difficult, but fair challenge.