An apparatus and method for ballasting and de-ballasting a vessel having a hull with several watertight ballast compartments. In a first embodiment, a pressure tank is isolatably and fluidly coupled to a common sea chest, to the atmosphere, to ballast compartments via a distribution manifold, and to a source of compressed gas. The pressure tank is first vented and filled with water from the sea chest, then isolated. The filled tank is then coupled to a ballast tank and the source of compressed gas, which displaces water from the tank to the ballast compartment. In other embodiments, ballasting is by venting and flooding compartments using individual sea chests located within the ballast compartments or a firewater system. For de-ballasting, compressed gas displaces ballast water through overboard discharges, through the common sea chest via the pressure tank, or through individual sea chests located within the ballast compartments.