There are two good places to sail on the Potomac. In Dahlgren, there is s public beach just north of the Rt#301 bridge on the Virginia side just past the visitor's center. The site does not have a concrete ramp so you may need a 4WD to securely get over the 100" of sandy beach. It is a good place for a day sail. Early morning and late evening there is a lot of wildlife in the area. It has restroom facilities and is frequently patrolled by King George county and Maryland DNR. (Maryland owns the river and if you are below mean low tide you are in Maryland.) There are no user fees. During warm weather it is best to get there early as the beach front parking spaces can fill up by 9:00am. The Potomac is 1.5 miles wide at this point.
Another location is Westmorland State Park which sits directly off Rt.#3 a few miles east of King George. It's hilly, densely wooded and very scenic. There is a parking fee of $3/car/day and a $4 boat fee to use the ramp. The park has great facilities and some nice cabins with great views if you want to stay overnight or longer or camp. They may even have a swimming pool. The river is about 5 miles wide at this point. Generally, the closer you get to the bay, the better the wind. The ramp is wide with a dock on each side and lots of parking for trailers. Across the river on the Maryland side is Breton Bay, St. Clement's Bay and the Wicomico River. The last time there I sailed my Adventure Island south along the Virginia shore past the cliffs, through Currioman and Nomini Bays, across to St. Clement's Island (historical site, has a large dock), around the various islands, the lower Wicomico, Cobb Island and back to Westmoreland State Park. This was about 8 hours in 5 to 15 knot winds. Lots of scenery. Plenty of quiet places to stop for lunch. Good for photos.