A hollow ballast tank contains within it a plurality of flexible bags for storing fluids such as fuel, freshwater and grey water. These fluids serve as functional ballast weight to complement the ballast weight provided by seawater which fills the remaining volume of the ballast tank upon being drawn into the tank through an intake pipe. Hoses run from each bag and emerge from a hatch in the upper surface of the ballast tank to service the boat. As fuel and freshwater are consumed, seawater is drawn into the ballast tank to compensate for the lost volume. Grey water may also be returned to a bag in the tank to serve as ballast weight.