Shark Attack Deathmatch
is $1 and a lot of fun.
Basically, it's deathmatch with scuba gear and spear guns. And sharks. Great whites circle the area. They'll often leave you alone. But if they smell blood, they'll come running. The sharks are animated pretty well, too, so it's creepy. There's a solo survival mode (which is all you get to try in the demo), which is just you trying to take out as many sharks as you can, and it's really tough because they take three shots or so to kill and the first one sets them after you, trailing blood. So by the time you kill it, you have to get the hell out of there because his buddies are coming.
Multiplayer is what this game is designed for, though, and they've done a decent job of it. There are lots of nooks and crannies in coral and derelict ships. There are a very limited number of oxygen and health power ups available, and they're in the same spot, which means you have to be desperate to go for them, but sometimes you are just that desperate.
The peripheral vision is a little limited by the mask, but you do get music cues when enemies are around. However, the chances are decent that someone won't see you before you get the chance to knife them (or I should say before you get knifed by them, as I never quite figured out how to get the knife to work, but was on the receiving end a number of times).
It's far from perfect and is rough around the indie edges. Long distance aiming is a little tricky. You'll see someone swimming in front of you and someone else behind them when suddenly the first person just dies, for example. Sharks do sometimes just appear. The game definitely needs more maps, not to mention more players.
But given all of that, for a buck, there's a lot of tense fun to be had.