Cousin of my wife's rented one on the water in Kingston, Mass. (next to Plymouth, not far from Boston and Cape Cod) last summer for her husband's 75th birthday. All the kids, grandkids, etc., came - more than ten, IIRC. Cost less than $4K/wk, though I don't remember the exact number.
We didn't stay there, since we live less than an hour away, but we visited. There was plenty of room for the entire gang.