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 light tonnage, Submarines, Destroyers, Light Cruisers, Light Carriers, or Support Ships from the second world war or before. You will have the appearance of a girl even if you were not before. Light ships tend to look on the younger side.