American River Taxi will provide water taxi service starting at the end of March 2012 during the Cherry Blossom Festival. Boats run between Georgetown, the Southwest Waterfront, and Diamond Teague Park, located behind Nationals Ballpark (the new riverwalk trail is now open to walk from Teague to the Yards Park). A weekly schedule and taxi route map are available on the American River Taxi website here.
Potomac Riverboat Company runs a Baseball Boat during Nationals home games between Alexandria and Diamond Teague Park, located behind Nationals Ballpark. Cost is $24 for a round-trip voyage or $15 for one-way. Potomac Riverboat also runs a service between National Harbor and Alexandria, where people can connect to the Baseball Park. Cost is $30 round trip or $20 one way. More information here
Diamond Teague Piers are managed by Gangplank Marina and can be contacted here .