Seaquest
Seaquest is an underwater shooter designed by Steve Cartwright and released by Activision in 1983. You command a small submarine, pick up divers, fight off sharks and enemy subs, and keep an eye on an oxygen tank that is always running down. The mix of shooting and rescuing, and the constant question of when to surface, keeps it interesting long after the first few minutes. You can play it here in your browser, with nothing to install.