I have lived in Gloucester for 25 years, windsurfed now kiting. The Eastern Point meter reads at least 5 knots higher than surface speeds due too its location high up on a tower. It has to read at least 20 knots to even rig a 14. We use Isle of Shoals for a better source of northely wind speeds on the north side of Cape Ann. Plum Island meter is not accurate in North to East winds as well. Coffins Beach is all private with no public launching at all, it is more or less a gated area.
Be careful in an outgoing tide, next stop Isle of Shoals or Halibut Point. Really only good in N-NW winds but if they drop, your downwind landing may be the rocky shore of Annisquam, again all private property.
THE spot close to Boston in a Northwest. Look at the Eastern Point iKitesurf meter for a true reading. Not really an option Memorial Day to Labor Day unless it's really cold and the lifeguards are feeling generous.
The launch area is between a large cliffside and rock outcropping so launches can be tough - look out for turbulent winds and spot a downwind launch beach area if you lose your wind... also look out for the rock outcropping that juts out toward the large buoy.
At low tide the tidal flats were great fun though....