The main conference boat is the Thomas Leighton, which sails from the dock of the Isles of Shoals Steamship Company in Portsmouth, New Hampshire. Click here to see the dock on Google Maps.
If you are arriving or departing at an odd time of the week, another option is to take the Uncle Oscar, a smaller boat out of Rye, New Hampshire.
Information on boats, travel, and parking can be found on the Star Island website.