The good news is that The Whitsundays are in fact part of the Great Barrier Reef Marine Park. Unlike visiting the reef from Cairns where the ocean is unprotected and often quite violent, The Whitsundays has been given shelter in the form of islands.
The Great Barrier Reef Marine Park includes the fringing reefs that hug most of The Whitsunday Islands. While these reefs are very different in the way they have formed, they are very similar to snorkel and dive.
Both fringing reefs and the outer reef have coral, fish and other marine life to swim around with.
On the outer reef, you are more likely to see hard corals and larger predators like sharks.
On the fringing reefs, you are more likely to see turtles and soft corals.
Each type of reef has its own unique structure and marine life, how good the snorkelling conditions will be really depend on the weather on the day.
Fringing reefs being protected by islands will almost always be a better choice for snorkelling, especially for families and poor swimmers. The other advantage of a fringing reef is the ability to enter the water from a beach, rather than jumping off a boat into deep water.
Currents are something to be aware of when snorkelling the outer barrier reef as they can often move very fast which can make snorkelling difficult. Fringing reefs do not get strong currents like that.