Mushroom Rock is Hamelin Bay's closest reef dive to shore. Marinelife.org.au describes it as a 6-15 m site about 200 m - a moderately stiff swim - from the boat ramp, with a small overhang, narrow channels on its western side and good fish life. The overhang on the north-west side hosts colourful invertebrates, and the fish list includes buffalo bream, old wives, pullers, scalyfin, morwong, herring, wrasse, leatherjackets and cuttlefish, with gorgonian fans.

The rock is part of the cluster of limestone features at the southern end of the bay along with Peak Island and Mushroom Reef, which lies between them. The combination of overhangs, channels and gorgonians gives it more colour than the sandy wreck sites, and it can be reached without a boat by fit divers.

It suits Open Water divers comfortable with a surface swim and moderate depth, and it pairs well with a snorkel with the stingrays at the ramp afterwards. Watch for boats using the ramp, and fly a flag during the swim out and back.

Hamelin Bay was a timber port from the 1880s, when a jetty built in 1882 and extended in 1898 served M. C. Davies' milling operations. It was notorious for wrecks in bad weather because it is exposed to the north-west gales of winter storms; the 22 July 1900 storm alone wrecked several ships, and only the old jetty piles remain today. The bay is now a recreation zone in the Ngari Capes Marine Park, and every wreck in it older than 75 years is protected.

Hamelin Bay is on the coast west of Karridale, about 35 km south of Margaret River and 30 km north of Augusta, reached via Caves Road and Hamelin Bay Road. Park at the boat ramp/jetty area beside the Hamelin Bay Holiday Park. Enter beside the ramp and swim about 200 m out to Mushroom Rock (a moderately stiff swim with gear, towing a float and flag), or anchor a small boat on sand nearby.

Sheltered inside Hamelin Bay in calm summer conditions; north-westerly winds and swell make the swim and the channels rough. General pattern on the Capes and Leeuwin coast: summer mornings often have light easterly (offshore) winds before a south-westerly sea breeze, and long-period south-westerly groundswell can arrive on otherwise calm days; winter fronts bring big swell and north-westerly winds. Check the Bureau of Meteorology swell forecast, not just the wind.

Swim out to the rock, descend on the north-west side to the overhang with its colourful invertebrates, then work through the narrow channels on the western side and around the rock at 6-12 m before swimming back. Agree a turn pressure before descending (half gas, or a third where current is likely), carry a DSMB, and make a 3-minute safety stop at 5 m.

No operator; self-guided from shore or private boats. Nearest dive services: Swan Dive (Busselton/Port Geographe) for fills, hire, nitrox and charters; no dive shop in Margaret River or Augusta.