The rusting hull at the entrance to Heron Island's harbour is one of the most historic vessels in Australian naval history. HMCS Protector was built by Armstrong, Mitchell & Co at Newcastle upon Tyne and launched in 1884 for the colony of South Australia. She was a 920-ton, 180 ft (about 55 m) flat-iron gunboat, and served in the Boxer Rebellion in China in 1900, was present in Sydney at Federation, and in the First World War took part in Australia's first overseas naval operation, escorting submarines during the capture of German New Guinea.

In the Second World War she was requisitioned for service in New Guinea but was damaged in a collision with a tug near Gladstone in 1943 and abandoned. Heron Island's developer, Captain Christian Poulsen, bought the damaged hull for £10 and had it towed to the island, where it was deliberately placed as a breakwater while a channel was blasted into the harbour. Today the hull is badly deteriorated; its teak deck has collapsed, and authorities warn against swimming close to it because of sharp edges and falling debris.

Ecologically, though, it is thriving: researchers describe the wreck as a haven providing shelter for fish, sharks, turtles and rays, and snorkellers at high tide see schools of fish, reef sharks and rays around it. At low tide the wreck can be approached on foot across the reef flat, and it is one of the most photographed sights on the island. It is not a scuba dive but an easy snorkel and a remarkable piece of history for anyone staying on Heron.

Heron Island is 72 km north-east of Gladstone. Fly or drive to Gladstone, then take the Heron Islander ferry from the Gladstone Marina terminal (72 Bryan Jordan Drive; check-in from 7:45 am, departs 9:30 am on scheduled days, about 2 hours), or transfer by seaplane or helicopter. Dive gear is carried on the ferry in addition to baggage.

On the island, the dive centre sits beside the harbour; boats leave the jetty for three dives daily and most Heron Reef sites are 5-15 minutes away. The wreck lies at the harbour entrance beside the island, a few minutes' walk from the resort. At low tide it can be reached by walking across the reef flat; at high tide it is snorkelled from the beach.

Check the resort's current guidance, as authorities advise against swimming close to the structure.

Heron Reef sites are chosen on the day by wind and tide. The prevailing south-east trade winds (strongest April-September, often 15-25 knots) make the southern and south-eastern reef edge rough, so the sheltered northern and western sites are dived most. Northerly winds in spring and summer reverse this.

The tidal range is up to 3-4 m and currents run strongly around the reef edge and through Wistari Channel on springs; visibility is usually best on the incoming tide. Cyclone season is November-April; summer storms and plankton blooms can drop visibility to 10 m. Stingers are uncommon this far south but present in summer.

The wreck sits on the reef flat at the harbour entrance, so it is only snorkellable around high tide; at low tide it is high and dry and can be viewed on a reef walk. Currents run through the harbour channel on the tide change, so snorkel near high-water slack and stay out of the channel.

Snorkel from the beach near high-water slack, staying well outside the hull and away from its sharp edges. Circle the wreck at a distance, watching the fish, sharks and rays sheltering around it, and return before the tide turns. Do not enter the harbour channel.

Snorkelled independently by Heron Island Resort guests; the dive centre and resort activities team can advise on tides and current safety guidance. Not a scuba site.