I work in Arlington and liveabord. We lived at Tantallon Marina for a year and half and had a 30-50 minute commute (traffic dependent). Tantallon is on Swan Creek off the Potomac about 5 miles south of the Wilson Bridge (MD side). Its cheaper than Gangplank and they have space available. Nice people there, but there are no ammenties except pumpout, electric and water. Nice new floating docks. We just moved a few weeks ago to Annapolis. My commute is a little longer and rent is higher but the sailing space is worth it. Eryka's post is accurate re Annapolis. She's a neigbor now
We checked out Herrington Harbor. Very nice resort style living, but its seems farthest from eveything.
Deale is much the same.
If you want to simply liveabord, I recommend looking close to DC. If you want to sail on the weekends or after work I recommend commuting from the Annapolis area.