Castle Rock is the granite landmark that gives Castle Bay its name, and the rocky outcrops around it are among the best snorkelling spots in Meelup Regional Park. Meelup Park describes Castle Bay's spectacular backdrop of Castle Rock with rocky outcrops that host an array of marine life, suitable for snorkelling and diving, and notes that winter surfing takes place there in strong swells. The Margaret River region's safety guide names Eagle Bay, Castle Rock and Rocky Point for their beautiful rock formations, suitable for snorkelling and freediving, and a south-west snorkel guide calls Castle Rock more rugged and secluded than Eagle Bay.

The granite boulders form gutters, overhangs and small swim-throughs in shallow water, with weed, sponges and invertebrates on the rock and seagrass between. Fish life includes wrasse, western blue devils, buffalo bream and old wives, and octopus and crabs are common in the cracks.

It suits snorkellers and shore divers of all levels in calm weather, and is a step up in interest from the sandy beaches. Depths are modest, so scuba divers get long, relaxed dives. As with other Meelup sites, avoid it when a big winter swell is running into the bay.

Geographe Bay faces north, so Cape Naturaliste blocks most of the prevailing south-westerly swell and sea breeze that close the ocean coast; that shelter and the warm Leeuwin Current make the Dunsborough shoreline one of the most reliably diveable stretches of the South West, and winter north-westerly storms are the main thing that shuts it down.

From Dunsborough town centre follow Geographe Bay Road and Cape Naturaliste Road north-west; the Meelup Regional Park beaches are signposted off Meelup Beach Road and Eagle Bay-Meelup Road, 3-10 km from town (5-15 minutes' drive). Castle Bay is signposted off Meelup Beach Road/Cape Naturaliste Road about 5-6 km from Dunsborough; park at the Castle Rock car park and walk down to the beach. Enter from the sand and swim toward the rock and outcrops.

Sheltered from the south-west, but Meelup Park notes Castle Bay picks up winter surf in strong swells, so avoid it in big north-westerly winter conditions. Summer mornings are calm and clear. General pattern in Geographe Bay: sheltered from south-westerly swell by Cape Naturaliste; summer mornings are calm before the afternoon sea breeze, while winter north-westerly fronts roll swell and chop into the bay and reduce visibility for several days.

Enter from the beach, snorkel or dive around Castle Rock and the adjacent outcrops at 2-5 m, then return along the shore. Snorkellers should stay within easy reach of the exit point and swim with a buddy; scuba divers at these depths need no safety stop but should ascend slowly and surface near the float.

No operator needed - self-guided snorkel and shore dive. Swan Dive (Busselton) supplies hire gear and fills and can advise on conditions.