The Deep by winkdot for Atari 2600.

About: Having created your own one man personal sub with (patent pending) hydro torpedoes, you set out to explore the waters and collect deep sea treasure. Only problem is, the treasures are guarded by sea drones placed there by a mysterious and extremely wealthy mad man who lacks the sub technology to collect the treasures for himself. You have unlimited hydro torpedoes to destroy the sea drones, but only a limited amount of oxygen. Your oxygen indicator is located on the bottom right screen and will reduce as oxygen is depleted. Your oxygen sensor will omit a sounds as it ticks down.

Joystick controls the sub in eight directions.
Joystick left or right and fire button – fires hydro torpedoes.
Each drone destroyed is worth 100 points.
Each treasure successfully retrieved is worth 1000 points.

Successful treasure retrieval renews your oxygen supply (anytime your at the surface your oxygen is renewed). Also this will move your sub to the next treasure dive. Each treasure dive is deeper and more dangerous. You have three subs and there are five treasure dives. Avoid any obstacles with your sub. You must explore the depths, collect the treasure and return it to the surface without getting destroyed.