Thompson Boathouse just south of Key Bridge & Georgetown charges like ~$5 to use the dock. It's super easy, just walk in & tell them & they'll set you up fast.

Or... just stroll right by the boathouse, onto the tiny little Mile Marker 0 peninsula just past it/south of it, & blow up there. You can either launch from the beach (hard) or just drop your board over the (out) side of old lock 1 and hop on (not hard but kind of intimidating). At your own risk!

On the downside, to my knowledge, neither has a particularly great place for stashing the bag & extra gear (pump). Maybe the boathouse has a real recommendation/some way to help, but I haven't asked. I've kind of left mine, well, wherever, & so far it has never been an issue.

Please see other posts to insure you meet legal requirements. ;)

Inflatable SUPs are great. I love being able to bike my board around.