This is Robinson Crusoe beneath the sea. Play as a lone captain who's submarine has crashed in an unknown location in the deep sea. The wrecked submarine is not suitable for the length of time needed to survive. With the tools available, harvest resources from the see and build a longer term shelter. Don't run out of oxygen, don't starve, don't get killed by any hostile creatures. Craft weapons and equipment for defense of life and home and to go exploring. Gather and/or farm plants and animals from the sea.

This game could also be thought of as Minecraft with actual reasons to do things (a plot).