You're a fisherman, bobbing up and down on the ocean in your little boat. All kinds of strange fish are swimming in the water. Catch them by swimming in their direction and shooting at them, when you're close to them. That's how you collect points, allowing you to get upgrades to catch even more fish!
Press P if you want to interrupt the game for a while or if you want to check old hints. Use the arrow keys to move around and click to shoot. If you want to jump into the water from your boat, you have to press the up arrow, simultaneously with the left or right arrow.