I've dived a bunch of times in Acadia -- mostly on the Schoodic Peninsula, which is a part of the Park that's on the mainland side. Nice easy shore diving, depths offshore in the 30-40 foot range. Fish life not prolific (occasional sculpin) but the anemones, urchins, lobsters and crabs were entertaining. On Mt. Desert Island, there used to be a charter operation at Bass Harbor, which I took to dives around some of the surrounding islands. Don't think they've operated for 5 or 6 years. Regular charter boats seem hard to come by. I've always wanted to drag a kayak along to do some kayak diving -- so many little coves, nooks and crannies to explore. Anyway, I've really enjoyed diving up there. Good luck.