The goal is to destroy all objects floating in a body of water within the screen. To do this, you need to create a chain reaction of shots and explosions. The number of attempts is limited. Also, keep in mind the countdown timer, which doesn't reset when the level is restarted.
Click the ship with your mouse (if you're playing on a computer) or tap the ship image on the screen (if you're playing on a mobile device) to fire.