Abstract: A tank buoyancy compensator for an underwater breathing tank comprises a ballast boot having a tubular portion to receive the end of the tank remote from the diver's head. The ballast boot includes an end portion that includes weight retention pin for releasably retaining one or more annular ballast weights on the end portion. A connector is attached to the retention pin and is operable by the diver at convenient location on the ballast boot for releasing the retention pin to jettison the weight(s) from the boot, for example, when the diver experiences an emergency situation.