Hmmm just reread the OP...one of the party doesn't like going on boats of any size..... that really limits the options as the snorkelling is generally better on islands a fair distance from the mainland.

Islands except Phuket and Samui involve a boat trip to get to them. Don't think there's much decent snorkelling off those either of those places but have never been to them....maybe someone knows some good spots.

You might see a few fish in the water off Phra Nang beach in Krabi as Rob suggests but it does involve a short boat ride to get there as well....

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