Collect treasures in the deep sea and bring them to your mother ship. Refuel or repair your submarine and if you have enough money you can also buy upgrades for your submarine in the base. Collect letters and create word BONUS and you will gain some extra cash.
If your submarine is destroyed, clicking Continue does not mean that you have to start again, only your score resets and you lose one half of your money. You DO NOT lose equipment you already purchased.