This is about the hardest shooter I've ever played and, guess what, it isn't very fun. It's got nice graphics and all, and decent sound, but your fish swims so slowly, has such a huge hitbox and such weak weapons, and the enemy fish stream by so quickly, that you'll find your poor fish getting simply battered to death (you can take a number of hits--watch the color of the eyes for your health level) in wave after wave of aquatic attackers.
I suppose that that game is unusual in a number of ways. The health meter display already mentioned, for one, and then again in tha the enemy fish don't shoot at all, they just move through the screen very quickly and erratically, in such massive schools that I can't really see how to dodge them, especially since your fish maneuvers like a cramped landlubber.
There also don't seem to be any bosses--at least, I haven't managed to reach anything like a boss. The bad fish just keep swimming across the screen faster than you can shoot them, or dodge them for that matter, and once that wave is done the next one comes right along. Whee?
Haha! Thanks to "masters'" insightful review on GameFAQs, I learned that your fish (it's a fish-shaped spaceship, apparently) heals slowly while not firing. Since your weapon is near useless, the best thing to do is just to dodge the enemies without firing back, so you'll heal continuously. Of course, the second boss (there are indeed bosses!) came and sat on me for a second which took me all the way from healthy to dead, and the game certainly wasn't fun by any means, but this rather amusingly illustrates the crappy design of the game. What *were* they thinking, really?