We stayed aboard in Charleston last winter, decent weather, nice town. We were at Ashley and they have a liveaboard time limit (no more than 3 months, IIRC), so that might not work... but the Megadock is just next door and might be suitable, and there are other marinas in the area.
Osprey Marina in "Myrtle Beach" is very pleasant and cost-friendly, nice folks... but it's sorta out in the boonies, not immediately near Myrtle Beach proper. Still, with a car, it could be a great place...
South River, Chesapeake Bay