The Aquarium is one of the South West's best-known secret snorkelling spots - secret no longer, as several guides note it has become crowded. Perth Is OK describes a small lagoon protected by large granite outcrops between Smiths Beach and Canal Rocks, reached by a 1.3 km walk from a small car park on Canal Rocks Road, with tranquil water ideal for snorkelling and swimming and rock pools full of crabs and other creatures. It warns that there are no toilets or water, that the southern end is more exposed to swell, and that there are sudden drop-offs in places. A south-west snorkel guide describes the trail as laden with tree roots, sand and rocks and the site as secluded and not universally accessible.

The appeal is a sheltered pool of clear water in the middle of an otherwise exposed surf coast. Among the granite boulders are wrasse, juvenile fish and crabs, with weed and seagrass on the sandy floor. It is shallow and calm enough for confident children, but the walk and the remoteness mean it suits reasonably fit visitors.

It is a snorkel rather than a scuba site - the carry is long and the water shallow. Visit early to avoid crowds, carry water and follow leave-no-trace principles.

The Capes coast between Cape Naturaliste and Cape Leeuwin faces the Indian Ocean and is famous for its surf, which is exactly what makes diving here so condition-dependent. Long-period south-westerly groundswell can arrive on calm, sunny days, and the regional ocean-safety guide advises observing a site in different conditions before diving it, never diving alone and flying a flag near boat ramps.

From Yallingup drive south on Caves Road and turn onto Canal Rocks Road; park at the small car park north of Canal Rocks. Walk about 1.3 km north along the coastal trail, which has tree roots, sand and rocks - carrying scuba gear is impractical, so this is a snorkel. The lagoon is enclosed by granite boulders; enter from the sand or low rocks at the sheltered northern end.

The granite shelters the lagoon from most swell, but the southern end is more exposed and there are sudden drop-offs in places. Only snorkel on small-swell days with easterly winds. It is increasingly crowded in summer.

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.

Enter at the sheltered end, snorkel around the lagoon and its rock pools, and keep away from the more exposed southern end if any swell is running. 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. Oceania Experiences runs small-group snorkel tours on the Capes coast; Swan Dive (Busselton) supplies hire gear and fills.