The goal:

You gotta go as deep as you can survive in an ocean cave, collecting coins to buy weapons on the StoreOS computers along the way, to defend yourself from hostile ocean creatures

The weapons:

Bombs: Clear all enemies on screen

Harpoon: Shoot it to kill enemies

Lethal Jellyfish Launcher (L.J.L™) : Chooses an enemy to kill instantly

The controls:

X - Use available weapon

Z - Exit menu's

Arrows - Move the player