Since a person on this forum posted passionately about Hinchinbrook Island, I looked into it. However, we are mad for snorkeling and here is the response from a person at the resort on Hinchinbrook (the Forum person sent in a link showing there was snorkeling on the island, but as you can see from the message, there really isn't.)
From Hinchinbrook resort: "The reason we do not have a great deal about snorkelling on the website is because we do not highlight ourselves as a snorkelling destination. Certainly a very brief outing to Brook Islands is on offer weather permitting- and you are right we have only continental reef, so really not exampling the Great Barrier Reef, and its for all those reasons that we try to deter anyone coming to us specifically for a snorkelling experience. Other Island and coastal cities eg Cairns or Port Douglas, offer by far the best opportunities to get to the reef
The real draw card for Hinchinbrook Island is its vastness and totally untouched wilderness - adjoining 2 world heritage areas."
So, my advice is to look at this island if the loveliness of the island is your prime interest, but look elsewhere for snorkeling.
Sally in Seattle