The Moltke is Magnetic Island's best-known and most frequently dived wreck, and a staple training dive for Townsville and island dive schools. The iron three-masted barque was built in Hamburg in 1870, around 50 m long, and arrived in Townsville in 1890. After years as a working vessel and later a hulk she was scuttled in Geoffrey Bay early in the twentieth century (sources give 1911 or 1913) to act as a breakwater for the island's ferry landing – but she was reputedly placed in the wrong spot to be much use.

During the Second World War she was used for target practice. Despite that treatment, the hull is still fairly intact and totally covered in hard and soft coral. It lies in 5–8 m (depending on tide), about 100 m offshore from the old vehicle-ferry landing, with a mooring buoy on the bow and a wooden pylon at the stern.

Around it stand coral heads and bommies, plus a research 'experiment platform' with metal arms and a large cylinder that the North Queensland Underwater Explorers Club uses as a start point. The wreck works as a fish aggregation. Schools of sweetlips, batfish and yellowtail fusiliers hang over it, with Moses perch, juvenile coral trout, drummer, rabbitfish, flowery cod and tuskfish; painted crayfish hide in the plates, nudibranchs crawl over the hull, and blue-spotted lagoon rays and epaulette sharks lie on the sand.

Wobbegongs, lionfish, moray eels and turtles are regular, and baby blacktip reef sharks patrol from the nearby mangroves. At night it is excellent, with crayfish and sleeping fish. The site is in a Marine National Park green zone.

Visibility is often poor and the wreck is easy to lose, so slow approaches, careful navigation and a guide on the first visit pay off. It suits new divers, snorkellers in good visibility and anyone after an easy, fishy night dive.

Magnetic Island is reached by SeaLink passenger ferry from the Breakwater terminal in Townsville to Nelly Bay (about 20 minutes, frequent daily services) or by the vehicle ferry from Ross Street, South Townsville (about 40 minutes). Buses, hire cars and 'topless' mokes run from the Nelly Bay terminal to Arcadia, Horseshoe Bay and Picnic Bay. From Nelly Bay drive or bus about 5 km north to Arcadia and turn down Armand Way to the car park beside the old vehicle-ferry landing at Geoffrey Bay.

Gear up at the car park and walk a few metres to the disused ferry ramp, which gives an easy concrete entry. Look for the large blue buoy offshore among the yellow snorkel-trail floats – it marks the Moltke (a wooden pylon marks the stern). Snorkel out about 100 m and descend the buoy line.

Boats may use the public mooring; no anchoring.

Magnetic Island's bays are sheltered from different winds depending on which side they face. The dominant south-east trade winds (15–25 knots, April–September) chop up exposed eastern and southern bays and stir up silt, so the island's best diving comes in the calmer months of August–December and on mornings before the sea breeze. Visibility is usually 3–8 m and occasionally 10–15 m; it drops sharply after strong winds, big spring tides and wet-season rain (January–April) when Ross River and Burdekin run-off discolour the water.

Dive at slack water, ideally high tide. Sargassum seaweed blooms in the hotter months can smother shallow reef flats. Marine stinger season (box jellyfish and Irukandji) runs roughly November to May – wear a full-length stinger suit or wetsuit with hood and gloves, carry vinegar.

Stinger nets are installed at Alma Bay, Picnic Bay and Horseshoe Bay in season. Geoffrey Bay faces east and is exposed to the south-east trades; tides and currents can be tricky here, so dive a calm day at high-tide slack.

Descend the blue buoy line to the bow, tour the hull slowly (the fish scatter if rushed), including the experiment platform with metal arms and the large cylinder that marks a start point, then visit the coral heads and bommies around the wreck. Return to the buoy on a compass bearing and ascend the line; swim back on the surface to the ferry ramp.

Pleasure Divers Magnetic Island (Arcadia) – guided shore dives daily at Alma Bay and Geoffrey Bay, PADI courses, gear hire; Adrenalin Snorkel & Dive picks up at Nelly Bay for reef trips.