If you want salt water, Washburn Island in Waquoit Bay is terrific. Good fishing in the pond (stripers, bluefish, fluke, and bonito and false albacore in the channel and right out front at the end of the summer.) There aren't too many spots, and they tend to book up quickly, but I really like it.
More info here:http://www.waquoitbayreserve.org/camp.htm