When you start a level, you'll see empty circles and circles with bathyscaphes on them. Tap any bathyscaphe it fires an air bubble in a fixed direction. Each bathyscaphe has its own angle. You can tap any of them, but the real challenge is to hit every other bathyscaphe with just one single bubble. That's the only way to complete the level.
The levels get harder and that's what makes them more interesting. You're also allowed to release several bubbles from the same bathyscaphe if needed. Plan your shots carefully, and victory will be yours.