Fancy a day on a private island in the Bahamas? This is the way to do it

Except there's more to this story than white-sand beaches, balmy waters and a pina colada or two (there are no less than nine bars dotted around the cay). Several cruise lines that ply the Bahamas and Caribbean have acquired private islands for the exclusive use of their passengers and Ocean Cay is one of the newer additions. Ten years ago it was an ugly scar in the Atlantic Ocean, an abandoned sand mine littered with thousands of tonnes of industrial waste, some of it buried - including whole excavators. In 2015, MSC Cruises signed a 100-year lease with the Bahamian government and set about turning the cay into paradise for its beach-seeking guests, spending $US200 million to do so.

But it's also a marine reserve, where research scientists are working to restore marine ecosystems, including crucial reef habitats, in collaboration with several universities. Passengers can laze around on a symphony of beach furniture - there are sun loungers, beanbags, day beds and cabanas - or take part (for a cost) in the many activities on offer, from paddleboarding and kayaking to stingray encounters and shipwreck snorkelling. All the while, hundreds of fragments of endangered coral are quietly growing in offshore nurseries. Ultimately, they will be "outplanted" across 165 square kilometres of the surrounding ocean.

"If you want to protect the fish, the turtles, the sharks, you've got to start with the foundation which is coral," we're told on a tour of the island with marine conservation biologist Dr Owen O'Shea, the MSC Foundation's Marine Research Program Manager at the time of our visit.

As we tootle about the cay on a golf cart, stopping at key locations, O'Shea points out a wetland that has lured more than 60 species of birds. Around the island, 5000 trees have been planted, along with tens of thousands of plants and shrubs, the vast majority of them indigenous to the Bahamas. Green iguanas - an invasive, unwelcome species, it turns out - are perched picturesquely on the rock walls that stabilise the shoreline, protecting it from the storms that sometimes slam into the cay. Post-tour, my niece and I deposit ourselves in a quiet corner of the island, on Sunset Beach. Given there are more than 5000 passengers on Meraviglia, it's remarkable how the cay absorbs all the humanity. Other spots are far busier, though - like family-friendly South Beach on the man-made lagoon, and Lighthouse Bay, which is adjacent to the ship's berth and has shark nets, the cay being amid a vast Bahamian shark sanctuary.

About 95 kilometres west of our sun loungers is Miami, and just a few metres behind us is the Cactus Bar, convenient for an afternoon cocktail as we lie beneath umbrellas, reading our books. Lunch is a lobster roll from one of the food trucks scattered about. A swim is compulsory, and in the distance waveriders are thumping across the water, another of the activities on offer. There's also a spa on the cay, where you can have a beachside massage and join a sunset yoga class.

But we're as idle as the iguanas, revelling in the tropical holiday vibe. Passengers can move back and forth freely between the ship and the cay, and after 4pm we head back to the Meraviglia for a gelato, then return to the island in time for a sunset rendezvous at the Lighthouse Bar - a fitting end to an excellent day in paradise.

The writer was a guest of MSC Cruises