You are the physical incarnation of a warship into human form. As a ship, you can walk on water as if it were land and move at the speed your ship half can (~35 knots). You have the displacement and radar cross section of your ship-self, on land you can consciously alter this to avoid destroying the floor.
At will you can summon your rigging which is a representation of your ship-self's equipment. Despite their small size, your weapons deal damage that can even exceed their full-sized counterparts. In addition to this, due to MSSB (Magical Sparkly Shipgirl Bullshit), you have minor mystical abilities that allow you to enhance your performance beyond your normal limits for a short time.
You are limited to ships of high tonnage, Battlecruisers, Battleships, and Aircraft Carriers from the second world war or before. You will have the appearance of a girl even if you were not before. Heavy ships tend to look on the older side.