Hit Points

From Deepolis Wiki

Hit Points (HP) for your Sub's Hull are shown in the bottom right of your game window. It indicates how many HP your sub currently has and the maximum amount it can have. Through the purchase of Armor Plating, you can increase the HP of your Sub. If you have 0 HP, this means your Sub has been destroyed and you have to repair it again or select a new Sub to continue playing the game.