Seaquest

Your divers have lifted a treasure from the bottom of the sea. On their way back to the surface they are attacked by hostile fish, sharks and enemy submarines. Therefore, it is your task to collect your divers with your own submarine. In doing so, you are not completely defenseless, because your submarine can fire bullets. But you have to watch your oxygen supply, which is displayed at the bottom of the screen - when it's depleted, you die. So you'll have to surface from time to time to replenish your oxygen supply. However, you will lose one of the rescued divers if you surface with less than six frogmen on board. After you have rescued six divers, you move on to the next stage. As the game progresses, the enemies become faster and more numerous.