Gravely Point Park is just at the north end of National Airport, south of the 14th St. bridge. If you come by car, it's off of the GW Parkway. The boat launch area is on the southwest corner of the park.
For G-Burg however, he won't be able to turn directly into the park as he's coming down the GW on the southbound side, where there's no left turn. If memory serves, he'll have to go past the park, exit into National Airport, and then follow the signs to come back onto GW northbound where it's an easy right hand turn.
G-burg, one word of warning, don't be cavalier with the speed limit signs in the airport area.