We swim past the chain locker, anchor and inside cargo hull two, where military gear including howitzer cannon, a 10-wheel truck, jeeps, steering wheels and tyres remain in their watery museum. It is my first shipwreck dive and I am hesitant. Nonetheless, I follow Jimmy, a dive master with Santo Island Dive and Fishing. As he points out guns, helmets and gas masks lying on the floor of the darkened hull, a loud rumbling stops us in our tracks. The noise goes on for some time. Jimmy makes a hand signal for us to get out of the hull as the water is muddied. I follow obediently and learn on resurfacing that we had experienced a 6.6-magnitude earthquake.

Vanuatu lies on the so-called Pacific Ring of Fire, a zone of frequent seismic activity caused by friction between shifting tectonic plates.

I remove my mask and laugh nervously. We survived, "shaken but not stirred", and I loved every moment crawling over this amazing ship.

Power opened up diving of the renowned wreck, thought to be the largest and most accessible in the world. The Coolidge lies just off the shore of Espiritu Santo – Spanish for "the Holy Spirit" – most commonly called Santo. It is the main drawcard for diving in Vanuatu and is considered the Pacific's premier wreck.

The 22,000-tonne liner, converted to a troop ship, sank fully laden during World War II after striking a mine laid earlier to thwart entry by midget submarines. More than 5000 troops made it to shore, but two men were killed. Within 90 minutes of being run aground, it was lying on the seabed with medical supplies, field weapons and motor vehicles.

We swim along the promenade deck and see the ship's massive gun, rifles, helmets, ammunition and a 1940s US Army field cooker. On deeper dives you can swim through the first-class dining saloon and visit the ceramic figure of a lady and her unicorn.

Other dives take you through the first-class lobby, library and continental lounge, with ornate Italian mosaic fountain, porthole skylights and brass mushroom-shaped ceiling lights, remnants from the ship's heyday as a luxury ocean liner built, with sister ship the SS President Hoover, for Dollar Steamship Lines. Apart from being an amazing wreck dive, the Coolidge has also formed a large artificial reef that's home to abundant marine life.

There is a reason the ship remains almost intact. Before Power had the ship declared a protected wreck in 1983, he used to tell divers they would cop a $12,000 fine if they were caught stealing from it, with spot checks at the airport. One couple, remorseful and fearful of the alleged fine, reportedly hid a blue cup they'd taken from the ship under their hotel bed, Power tells me.

Most divers come to Santo to explore the Coolidge, but there are other terrific dives and plenty more reasons for visiting Vanuatu's largest island.

After our dive, we head back to the beach and enjoy a lunch of tropical fruit and baguettes (a legacy of French colonisation), before diving Million Dollar Point. This is where thousands of tonnes of US military equipment was controversially dumped into the sea at the end of World War II. Just offshore, snorkellers and divers can see scores of coral-encrusted bulldozers, cranes, forklifts and trucks piled on top of each other at this unique dive site.

Possibly my favourite dive is of the USS Tucker, one of only two diveable US destroyers in the world. I dive with 23-year-old Vanuatu-born Australian dive master Mike Babcock, who grew up on an island just off Santo where his father worked as a linguist. He returned three years ago from Brisbane and stayed to set up Coral Quay Resort's dive operation.

The Tucker also hit a mine and sank 70 years ago, just before the Coolidge went down. It lies in three main pieces – the bow, the mid-section engine blocks, and the stern, spread across 100 metres of reef off Malo Island. Here, on what is an easy, enjoyable dive, we see dozens of species of tropical fish, swim inside parts of the wreck and spot a shy green turtle. Amazingly, we are the only divers there.

Later, perched on the back of a moped, I soak up the balmy air as we wave at friendly locals on a scenic drive along the island's east coast.

Sure, it is the Coolidge that bought me here, but I will return for what Santo offers in spades – a truly authentic slice of Pacific life.

Sheriden Rhodes travelled to Vanuatu courtesy of Vanuatu Tourism

Trip notes

Getting there: Air Vanuatu flies to Santo from Sydney via Port Vila daily. airvanuatu.com.

Staying there: A beachfront bungalow at the charming Aore Island Resort costs from $250, including continental breakfast. aoreresort.com. Rooms at Deco Stop Lodge, an ideal base for serious divers in the heart of Luganville, are priced from $173, including return airport transfers and continental breakfast. decostop.com.vu.

Diving there: Allan Power Dive Tours (allan-power-santo.com); Coral Quays Fish & Dive (coralquays.com); Santo Dive and Fishing (santodive.com).

More information: vanuatu.travel.

Dive on in: three more experiences

1 Take a swim in the amazing Blue Holes with its staggeringly beautiful fresh water the colour of sapphires and have a picnic at the postcard-perfect Champagne Beach.

2 Take a tour to see relics of World War II, including Quonset huts, three bomber strips and the Palikulo US seaplane base.

3 Learn how the Banks Islands people live, self-sufficiently and dressed in clothes woven from the leaves of the jungle, at Leweton cultural village. You will see demonstrations of unique water music originating from the islands of Guau and Mere Lava.