Swimming at Whale island is wonderful.. perfect water, very clear and calm and no jellyfish, at least in July. The water is neither too shallow nor too deep, and there's no boats are aquascooters to worry about. it's really suitable for all, including children and non-swimmers.
Equipment: Bring a swimsuit, high factor sunscreen and not much else. Towels are optional, as you can rent them on the island.
Whale Island is a great place to snorkel... you can join the morning snorkeling boat trip or else you can snorkel right from the island's beach, where there are still plenty of fishes and corals to see without going far.
The added bonus of not joining the snorkeling trip is that you will have the entire island to yourself.. most people come here to dive, and those who do not dive normally go on the snorkeling trips... so on most days we had the place to ourselves.
Equipment: If you have a snorkel set, bring it with you, or else you can rent it on the island. Do bring a water camera, though, because it is not on sale.