Arcade Game: Sea Fighter Poseidon (1984 Taito)

A Shooting 'em up underwater game where you start on the dive propulsion vehicle that can shoot torpedoes. You mission is to rescue all hostages and to avoid and shoot all enemies that comes your way to attack you. If your vehicle almost runs out of fuel, you can avoid this by changing vehicles by hitting the dismount button to get off the current vehicle. You now swim freely and your weapon is a powerful harpoon. If you kill the owner of the another dive vehicle, you can mount it and use torpedoes again. The number of hostages you rescue increases as you move the the next level. When all hostages are rescued, the level ends.

Note: No cheats used at all. Only savestates used. The game is endless as it restarts the 1st level after you've beaten the last level so I quit afterwards.