try Catseye Beach? which was OK IMHO and iirc. If you can afford the money
and the time snorkelling actually on the reef or around some of the other
Islands is better. Saying that any snorkel around a bunch of rocks or a
structure will be interesting , probably rewarding and very different
depending where you come from

Whitehaven beach proper is pretty much a waste of time for snorkelling.
Catseye (the main resort beach) is surprisingly good - go once at high
tide and once at low. Day trips to the outer reef are long and arduous,
and IMHO not particularly worth it for snorkelling if time is short but
very much worth it for scuba. Butterfly Bay, not far from Hayman Island
is simply wonderful for snorkelling.

People will tell you the Whitsundays are submerged mainland islands, not
coral cays and therefore not actually part of the barrier reef. That's
true, but there's still good fringing coral in places and associated
fish life. Definitely worth packing your snorkel.

-- Ian