Four miles south of Boothbay Harbor on Southport Island, Gray Homestead has 40 wooded RV sites (some oceanfront, all with hookups), where you can also pitch a tent. There's swimming at the beach and kayak rental. There are a couple of condos and cottages for rent, too (weekly in summer; $700 to $1300).
Southport Island is connected by road to Boothbay Harbor; take ME 27 then ME 238 to reach Gray Homestead.