If you're hoping to boondock and beach play.. not much (if any) to be found. If looking for commercial camping besides Acadia, I have two I like that have a beach and can be fished from shore or dock (boat launch also). Both are tough to get into 'in-season'. So yes, they're popular.
First one is: Winslow Park just south of Freeport. Very nice community park. Only con to me is they do allow generators (within certain times). I have spoken to them about designating an area for non-generator.. They feel it will reduce their occupancy.
Second one is: Lamoine State Park. This is across the narrows from Acadia. Far less traffic and very easy drive if you so wish to run into Acadia NP. Shore sites are full most all the time, including off-season. Nice dock to fish from, great lawn areas with water view and beach area.
I look for places that have some space (privacy) yet an open enough feel. I'm still exploring the Maine coast for spots. I like using iOverlander to scout for possible's.