Living in the town next to Salem, I have never been a fan of Winter Island but I know some are.  It is next door to a power plant and the waste water plant where you can dump.  I guess I have not taken to the train lately as I remember the family use to go in and out of Boston for $20.  Boston Minuteman is nice with a train nearby to Boston. If in Salisbury, you can get a train in Newburyport that goes to Salem.  One last choice, Gloucester has a campground as well, never been, as well as a train to Salem.