Eight young men slink around charcoal fires in a thatched spirit house in Yentchan village, on the Sepik River in Papua New Guinea, wearing only a few modesty leaves around their waists and caked river mud on their heads.

Members of the Iatmul people, they have just been initiated into the puk-puk or crocodile culture – a ritual where the skins on their backs and shoulders are cut with razorblades to create a decorative stippled pattern resembling that of a crocodile, from which they believe they evolved. Their wounds are then pasted with ash and river mud and pummelled with bamboo bats while the heat from the fire almost tans their wounds like leather. This tradition has been passed down from generation to generation.

Being privy to this extraordinary scenario is a highlight of the 10-day Bismarck Bonanza cruise on the 36-passenger luxury adventure vessel True North. Helicopter pilot Rob Colbert had flown us over flooded villages and sago plantations before landing next to the hut where the initiates greeted us with an energetic drumming dance.

It’s one of the great advantages of being a passenger on board a vessel that offers a three-dimensional opportunity to discover the local environment: from the comfort of the air-conditioned boat, from the air in the six-passenger Eurocopter and underwater – snorkelling and diving in the company of marine scientist Dr Andy Lewis. True North was purpose-built for exploring river systems and coastal waters and, because of her size and shallow 2.2-metre draft, she can venture where many other vessels cannot.

Having always been fascinated by this exotic country, I didn’t need too much convincing to join 30 other passengers on this inaugural sailing along PNG’s north-east coast. We were to travel 70 nautical miles down the art-rich Sepik River and call in to rarely-visited, remote island communities.

Following a two-hour charter flight from Cairns to Madang, we board True North, sip coconut water from flower-trimmed coconuts and settle in for our first night afloat under the care of the 18-strong, all-Australian crew. Chefs Nikholas Flack and Zac Johnson prepare tender kangaroo fillets with preserved hibiscus flowers as canapes. Dinner is pan-seared scallops on pureed celeriac followed by pistachio cake with apple sorbet, kiwi fruit and Italian meringue. With experience in the Melbourne kitchens of Shannon Bennett’s Vue de Monde, the talented pair produce outstanding meals and make fresh bread, yoghurt and ice-creams daily.

Accommodation is in 18 comfortable suites. There’s a spacious dining room, welcoming lounge bar with comfy sofas, a rear deck for casual dining and drinks and a well-equipped transom that becomes a hive of activity each morning and afternoon as fishing, snorkelling and scuba diving get under way aboard six aluminium-hulled expedition boats. Each day’s program begins with breakfast at 6.30am.

We visit villages with lively produce markets and displays of intricately carved artefacts including masks, story boards, coconut shells, grass skirts and bilongs – simple string bags that locals carry over their shoulders or across the tops of their head, and are so called because everything inside “bilong” them. Other villages stage lively sing-sings to welcome us, the whole population dressing up in elaborate beads, feathers and, in one village, as giant crocodile puppets.

Leaving the Sepik, we cruise overnight to the far-flung Ninigo Islands – a large open atoll containing smaller atolls and some 50 small islets. After lunching on plump Sepik cherubin prawns, we set off to snorkel the Pelleluhu Passage.

When we return to True North, the ship is surrounded by a flotilla of dug-outs roped together as each fisherman drops off his catch of spectacularly coloured “painted” crayfish. The chefs weigh and buy about 60kg (about 100 crays) for $7 a kilo – many of them relegated to the freezer for future trips. During the cruise, seafood includes elusive black bass caught by locals and scale fish caught by guests – Spanish mackerel that ends up crumbed as fish and chips, barracuda for a classic Nicoise salad and yellow-fin tuna for sashimi.

One day, we come across pods of pilot whales that breach and splash playfully; another day we are surrounded by hundreds of spinner dolphins that swim under the tenders. And while snorkelling and diving, we spy turtles, manta and eagle rays, white-tipped sharks, schools of black and red snapper, colourful parrotfish, angelfish, vibrant blue damsel fish and various tiny striped anemone “Nemo” clownfish, for which the area is noted.

In the wheelhouse, Greg Dunn is enjoying his second stint as master on True North, having skippered luxury maxi-yachts around the world for the past few years. With its helipad, sonar, radar, large freezers that can keep frozen goods for months, a walk-in fridge the size of a small bedroom and the capacity to produce 36,000 litres of fresh water a day, Greg declares the boat was built for adventure. “It’s an adventure ship in every way,” he says. “How many other boats can send their helicopter up to look for a new dive site?”