Seaquest

Your divers have collected treasure from the ocean floor. On their way back up to surface they are attacked by hostile fish, sharks and enemy submarines. It is, therefore, your task to collect your divers with your own submarine. While doing that you are not totally defenceless since your sub is able to fire bullets. But you have to watch out for your oxygen level which is shown at the bottom of the screen - once it is depleted, you die.