This game is all about exploring, collecting and upgrading.
Use the arrow keys to steer your submarine to dive and collect coins, points and search for the 10 ancient relics. Return close to the surface to replenish your air supply and to exit your submarine. You can upgrade your sub by collecting coins.
I know the game may get a bit boring, but it was a great exercise for me in developing an upgrade system and how to apply that to gameplay. I hope you like it (for what it is!)