Tucked into a rocky cove on the northern side of Arthur Bay lie the remains of an old steam dredge. Local sources have long called it the Platypus, Townsville's first channel dredge, used to deepen the harbour approaches – that is the name the North Queensland Underwater Explorers Club uses. The Marine Life Network argues that it is more likely the iron bucket dredge Octopus, built in 1882 and scuttled in 1928; the club's own wreck index records the Octopus as scuttled six years after being beached in Nelly Bay and gives its location as off Desolation Point.

Either way, the identity is not settled, and divers should treat the name as provisional. What survives is the machinery: the hull has largely broken up, but the huge square steam boilers and runs of steam plumbing remain, encrusted with hard coral at 3–7 m, hard against the shoreline. Fish use the structure heavily – the Marine Life Network lists large schools of angelfish, sergeant majors, parrotfish, batfish and trevally – and the surrounding granite boulders and coral add interest.

Finding it is part of the dive. By boat, locals use the low point of a small hill above the cove and a large flat rock as marks. From shore, divers swim from Arthur Bay beach along the left-hand shoreline towards the point.

The road to Arthur Bay is unsealed and often in poor repair, so many divers go by boat. It is a shallow, easy dive for Open Water divers, rarely visited and with a pleasing sense of discovery. Visibility and sargassum are the usual Magnetic Island caveats, and the site is best on a calm, clear morning in the cooler months.

Boat is easiest: launch at Horseshoe Bay and run east round to Arthur Bay (10–15 minutes); the wreck lies in the rocky cove past the bay, identified by the lowest point of a small hill above the cove and a large flat rock used as a reference. By road, take Radical Bay Road from the Horseshoe Bay bus stop near the Forts walking track (an unsealed, often badly washed-out road) to the Arthur Bay car park, walk down to the beach and swim along the left-hand (northern) shoreline towards the point. Gear carry from the car park is 100–200 m on a track.

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. Arthur Bay faces east and is exposed to the trades; dive on calm days at high-tide slack. Summer sargassum blooms affect the bay.

From the large flat rock, descend at the base of the cove to 3–7 m, locate the square boilers and steam plumbing, circle them, then swim along the shoreline towards Alma Bay if gas and conditions allow before returning.

No regular operator; NQUEC club dives and private boats. Rarely visited. Nearest dive centres for gear hire, fills and local advice: Pleasure Divers Magnetic Island, Adrenalin Snorkel & Dive.