Sea Invasion is a Space Invaders variant set in the ocean. The player is a scuba diver who while down on the sea bed is being attacked by different sea creatures. The player moves left and right along the sea bed and has use his spear gun to shoot at the creatures above him while watching out for their projectiles. Some protection can be had from corals but these will disappear as they are hit by projectiles. There are three different sea creatures, octopi, crabs and starfish and the player gets points based on the position of each. Octopi are farthest away from the player and are therefore worth the most points. At times a shark swims across the surface and the player can get a bonus score for shooting it. The game is endless, when all creatures has been killed it all starts over with a new level until the player has lost all his lives.