Blast away ships by sneaking up underneath them and sinking as many of them as you can by using your torpedoes. You have a very limited amount of health and energy to use while underwater, so be sure to keep an eye on those indicators while sinking ships. Each level has an objective that you must meet in order to move onto the next level. If you start sinking enough ships you will earn money that you can use to buy upgrades like armor, speed, fuel efficiency, and torpedoes.