Though the Bora Bora Nui and St. Regis Bora Bora are better properties, I can't really imagine better snorkling anywhere in the world than the Sheraton Moorea which is still a Category 5 (though you can't redeem for an Overwater Bungalow).

Second the Westin St. John.

And I'd have suggested the Sheraton Port Douglas as a base for scuba diving trips if you hadn't just done the Great Barrier Reef (yes, I know this property is disfavored by some...)

Agree with your initial assessment that Khao Lak is out in June/July (though I was there a couple months back and the seas were remarkably calm, still it's low season and not many resort businesses were operating).