The intrusive drilling sound of our speedboat as we slice through the ocean cuts through the tranquillity as we zoom along to our secret destination. Some of Fiji’s 333 islands can be seen in the distance – they’re lush and green and with the sun beaming down, it’s like a scene straight out ofLost.

Our guide Tim nods at the driver who quickly shuts off the engine – the silence is like a jolt to the senses as we silently drift along looking for the right spot.

“Here,” Tim says authoritatively. We’re already kitted up in our diving gear, so we roll backwards and descend into the warm, crystal-clear water. The underground world comes to life before our eyes – coral of every pastel colour is highlighted by the sunrays piercing the surface. It’s teeming with life as schools of vibrant fish dart about, not in the least bit inquisitive about their huge intruders.

Tim and Simon fin along in front of me as I get distracted exploring the magical wonderland. As they turn to check I’m okay, I can see their eyes bulging behind their masks. They point behind me, motioning for me to turn around. I feel my heart pounding – I’m a keen diver, but I’ve never seen anything bigger or scarier than a trigger fish before, and I’m fearing the worst.

Everything seems to happen in slow motion as I turn to face a gigantic manta ray, spanning about five metres, gliding effortlessly through the water towards us. Sensing it’s got company, it veers off, moving too quickly for us to follow.

Back onboard the Tui Tai, we boast to anyone who will listen – the crew, all Fijians wearing loud shirts and salu-salus (flower necklaces) – have probably heard a similar story a thousand times before, but they’re polite enough to act as though it’s the first.

I boarded the vessel at Natewa Bay on Fiji’s north island Vanua Levu. Having been reluctant to do a cruise, I was assured this is the best way to explore some of the more remote islands and some of the best diving in the world, anchoring overnight in the South Pacific Ocean. But it’s not the type of cruise where you watch the world go by. Days start at 6am with yoga on the deck as the sun comes up. After that, guests – of which there are only 22 – can then choose to paddleboard, kayak, snorkel, surf, dive or go hiking and biking on an island.

Overnight, the boat is back on the move around the archipelago, so you wake in a new destination every morning. My group takes the bikes over to the volcanic Rabi Island. It’s inhabited by just 5,000 Micronesian people, most of whom relocated from Kiribati after World War II. Hordes of smiley children run beside us as we pedal along the small, dusty tracks, weaving in between tiny huts. Everyone on the island is friendly and before long, we are the VIP guests of the village.