There was this awesome submarine rpg type game for the pc about 10 yrs back. You would control up and down motions and such and drive around under water doing lots of quests etc. It was 3rd person so you would view behind the yellowish submarine, its kind of like aquanox but not quite. You would also have a grappeling hook or something to pick up various goods and weapons. There would also be a trade and money system, the world was pretty big and open and you had a world map. The world had underwater cities and stations one could dock to. And there were boss fights with bigger ships etc that were pretty massive.