Game of the Day - Battle Shark

Battle Shark is a great 1st person shooter that starts off underwater.  You use a cross-hair that floats/converges on vertical and horizontal lines to aim at enemies and incoming missiles/torpedoes.  The game originally used a periscope setup in the arcade to control the action.  Now (in MAME) you use a joystick (or possibly, a trackball) and a fire button.

You start with a fixed number of torpedoes so you have to use them judiciously.  You can earn more and you can increase your capacity for holding them as the game progresses.  One of the nice things about the game is the damage you take shows up on your screen - both in a damage gauge and as cracks in the window/glass that is your cross-hair sight.  You have to shoot enemies going across the screen left or right AND shoot torpedoes launched by those enemies that are headed right at you.  You have to "lead" the targets as it takes the torpedoes a little time to travel to the target.

The graphics are fun, especially the smooth way the targeting cross-hair floats across the screen as you move your joystick.  In later levels, the action surfaces and you wind up fighting enemies on, and then above, the water's surface.  It's a relatively unique looking and feeling game and will keep you occupied for a while.