This isn't a game starring Roy Scheider and a talking dolphin, it's an extremely fun rescue game from one of Activision's legends, Steve Cartwright.
The game description from the Seaquest game manual:

Your sub is heavily weaponed, which probably leads to the limitation of holding only six divers at a time - the rest of the sub must be packed solidly with torpedos!

You'll need it, because your divers are in very dangerous waters. Schools of mutant, man-eating, sub-destroying killer shark chase your divers, while enemy subs hunt you down like you were the Red October filled with diamonds.

Oh, by the way, your life support systems have conked out, so while you're submerged you're using precious oxygen, which can only be replenished by surfacing. The enemy subs shoot back, and the sharks bob up and down, making them more difficult to hit.

When you surface, you will debark at least one diver, unless your hold is empty, which will destroy your ship.

I probably forgot to mention the surface ship looking for you after the second round... What a tangled web we weave.

There are a lot of different things going on, and that's what gives this game so much depth - you can play with several different strategies, and be successful in different ways.

For example, when you surface, the round increases, which means that there will either be more or faster enemies, but they'll also be worth more points. You have a large scoring potential in the divers, as their max point score is 1000 points each. So you might want to rescue one diver per round to quickly get the round number and scoring high, then start going for killing the enemies and six divers - you'll get a new sub almost every round.

And that's another great thing about Seaquest, it is very well balanced, once you get in a groove, you can play for quite a while, but it doesn't get boring, because it keeps getting harder. But the harder it gets, the faster you get extra subs.

My recipe for success is to go for six divers per round, and use up nearly all of my oxygen killing enemy subs and sharks. My high score to date is 137,260 - not too shabby, but I can do better. There's a sweet spot for each of the four "lanes" that the enemies / divers travel in - with constant fire, and getting into the sweet spots, you can lay waste to a lot of enemies, really upping your score.

This is one of my top ten favorite Atari 2600 Games of all time - great gameplay, great balance, and lots of fun! - check Seaquest out!

You can get the ROM and 2600 Emulators at the Classic Gaming Game Vault.