Graphics: Average. Nothing special. Nice to see the upgrades going on, but the levels could have done with more design to them - just having square, brown, featureless caverns is a little uninspiring. Nice lighting effects as you go deeper, around you and the fish.
Sound: Awful. That... I hesitate to call it Muzak, as Muzak has at least had some care and attention put into it... was just appalling.
Gameplay: Fun. Nice exploring, though it would be nice if you got a message once you'd found everything - a little congratulations or something. I ended up finding everything, and going and sitting by the Wreck on level 10 until my air run out - I hadn't died up until that point, so wanted to see what happened (Not much), and I thought it might trigger a secret. It didn't. I'd make it a little more challenging (Make crashing into the walls drop a tiny bit of air, or put kraken or sharks in it as enemies to avoid, something like that), and put a game-over screen.
Sadly, the game suffers from a lack of any ability to predict your route. At later levels, the ability to pass becomes more a matter of chance than ability, and this makes a good game a sadly unplayable mess. Add in some intelligence in the way things come, and I'll look forward to Version 2