Seaquest is another one of my all time favorite Atari 2600 games!  You control a submarine.  You try to pick up divers who are being chased by sharks, shoot sharks and shoot enemy submarines who shoot at you.  The sharks, enemy subs and divers travel across the screen either from right to left or left to right along one of 4 paths.  You can move in all directions around the screen.  When you collect 6 divers, you have to get them to the surface to unload before going back in for more divers.  You also have a limited amount of oxygen and must go to the surface when your oxygen meter is running low to renew your supply.  Each time you resurface without a full load of 6 divers, you loose 1 diver.  If you resurface without picking up any divers first, you lose a one of your subs.  This is a really great game, but a little bit more difficult to find than most.  Still, you should be able to pick it up for around $5.