could be utilized if desired each air valve assembly includes an air conduit 23 connected to an aperture in ballast tank 17 to provide a passageway for the flow of air to and from the ballast tank and a check valve 24 to prevent the entry of water into the air conduit check valve 24 preferably comprises a float valve that will automatically close and open in response to the presence or absence of water at the valve in the preferred embodiment of the invention air conduits 23 from the air valve assemblies extend to a common head 26 disposed at a location that will remain well above the surface of the water in which the pontoon is floating an illustrative arrangement is shown in fig5 when the sea valve assembly is opened and water flows into the ballast tank displaced air exits the ballast tank through the air valve assemblies until the sea valve assembly is closed when the operator desires to empty the ballast tank a source of pressurized air is connected to the air conduits preferably at their junction at head 26 and pressurized air is forced into the ballast tank raising the internal pressure of the tank the pressurized air may be provided from a compressed air storage tank from an operating compressor or a combination the sea valve is opened and water is forced from the ballast tank through pipe 21 until the ballast tank is empty or the draft of the pontoon has been adjusted.