One additional comment: take the plane. Cape Air has constant service during the high season. They run sections for most flights, and the sections are staggered so that there is a plane leaving every half hour, or so it seems.
Much, much more convenient than any of the three ferry services. And if you are flying into Logan, you don't have to get to the wharf in Boston.
And now we even have a magnetometer at our little landing strip in the dunes. (New development to allow Terminal A at Logan to be secure.)
If we could only get Cape Air to start a true frequent flyer program! (They term their discount books a frequent flyer program, but it is simply a flat price for ten vouchers.)