If you like a small-group experience, Coral Cats' Wanaka catamaran is a great choice for a day-trip to the Mamanuca islands.
The speedy cat heads out from Port Denarau towards the islands of Malolo Laila, Tavarua, Namotu, and out to the outer reefs. Snorkelers (gear included) can check out the amazing array of tropical fish.
A hearty barbecue lunch is served on the island of Malolo Laila, with time afterwards to loll on the white sand beach or swim.
The Wanaka's crew make this a particularly joyful trip, especially for children, sharing local lore and experiences.