The Submarine is a stealth-based naval unit used for covertly attacking ships and naval buildings (Naval Shipyard, Oil Rig, etc...). It is difficult to defend against due to only being able to be damaged by destroyers and other submarines.
Strategic Purposes / Usages
Due to it being required to be researched to be used, the submarine usually is created during mid-game. Submarines usually should be used against buildings nearby or on water and should avoid attacking a group of destroyers at all times as the submarine can only be attacked from them. Due to this, it would be logical to put in other ships into the mix such as battleships or cloak the submarine and flee to get the lowest amount of damage inflicted onto the submarine.
If a Submarine is the last unit remaining and it cloaks, it will instantly die to prevent it staying cloaked and unable to be attacked which would make the game indefinite.