@Glenn MacGrady, Here is a link to the Maine Island Trail Association:
https://mita.org/
@dcloots, I would be wary about undertaking exposed ocean paddling in an open canoe designed for rivers and lakes. They're not well suited to the environment.
I've done some sea kayaking on the Maine Island Trail. I vividly remember trying to desperately round a point, as an incoming wind swell arrived diagonally to the shoreline and forced us directly towards the rocky shore break. We had touring kayaks (with rudders!) and still had a heck of time keeping them pointed offshore. We would have been toast in canoes.
The prevailing winds in the summer are typically southwesterly 10-15 knots. With a long fetch, that can make for a tricky paddling surface under average circumstances, and the windspeed can pop up in a moment. So for any open passage, you'd really need a perfect day and be able to count on it staying that way. At the very least, I'd make sure you had a spray deck to reduce your windage and keep chop out.
Other aspects that are materially different than freshwater: The tides are going to pull on you significantly, so consult a current map. The fog can set up in an instant and is quite alarming and difficult to navigate. You'll be avoiding a lot of boat traffic too, so there's necessary equipment specific to the marine environment, like a handheld VHF, fog horn, running lights, emergency strobe and flares, etc. And the water is cold, year round.
If you pick your spots, time the forecast just right and keep the passages short, it could be great, but there's a lot more risk. While I would prefer to canoe almost always, I've come to enjoy my touring kayak in its own right.