My general recollection is that the anchorage at the Isle of Shoals is pretty open, it wouldn't seem like much of an over night destination. I'd skip Rockport too, you can save a lot of distance and have a fun evening, by heading straight to Glouchester via the Annisquam river. head over to Rocky Neck and have dinner at one of the restaurants, they may let you spend the night on their dock for free. try these guys The Studio, Rocky Neck, Gloucester, Massachusetts
Save some time to walk around.