I was also going to suggest a stop at Great Dog en route to North Sound. There isn't much snorkeling in North Sound... we've had OK conditions on the back of Colquhon reef.

Dinghy to the inside of the reef in Eustacia sound and look for a white BEYC buoy at the far end. There are a couple of sunk cannons that are not hard to find with 50 or 80 feet of the buoy.