This is a classic defender type game but with jellyfish. The goal of the game is to shoot as many of the Jellyfish as you can to unlock the next level and get a high score.
Use the up and down buttons to position the sub and the shoot button to kill the jellyfish. The first objective is to kill enough jellyfish to unlock the next level and the second is to get the highest score. Bullets that do not hit anything take points away from your score so use them wisely.