Bill,
I can't speak from experience about the south side of the Potomac, but I can recomend Point Lookout Marina on the north bank, Smith Creek off the St.Mary's to be precise. (6 NM from the actual Point Lookout, haulout, shop, and do it yourself or contract yard, resturant on site, wi-fi both from the marina and the resturant, pool, fixed docks that are somewhat rickety on the transient and fuel dock side but in good shape on "G" dock, where the long term sailboats stay. 2 foot tide range, crabs and nettles galore. The Ridge market, run by a Chinese family, is about a mile away. Lexington Park and Pax River NAS with all the associated suburban/semi-metro sprawll are about 10 miles distant. On top of all this, the price is good for a slip by the month or longer, and liveaboards are welcome.) We stayed there this July into August, using the marina as a refuge from the heat and daysailing to Leonardtown, St.Mary's, and Solomon's.