North-west of Mindarie Marina, off Quinns Rocks and Alkimos, lies a cluster of limestone reefs at 12–23 m that local divers have named over the years: the Coral Garden, Grouper Gully, Crayfish Gully, Dewy Ledge, Gordon's Grotto, Ian's Cave Reef, Paul's Point, Snapper Peak, The 23 Metre Ledge and The Wall, along with Chris's Cliff, South Pass and Staggie Reef. The Marine Life Network's north-of-the-river list gives GPS positions and brief notes for most of them, and it is the main published source for these sites.

Dewy Ledge is a reef ridge about 50 m wide at 15–20 m, and the Marine Life Network notes large fish populations. It lies a few hundred metres from Crayfish Gully and Snapper Peak in the deeper reef cluster north-west of Mindarie. The ridge form means a clear edge to follow on each side, which makes navigation straightforward: descend on the top, follow one edge, cross and return along the other.

Like the other reefs in the group, the limestone here is covered in kelp and red algae on top, with sponges, sea whips and corals in the shaded ledges. Typical fish of this part of the coast include buffalo bream, western blue devils, old wives, bullseyes, wrasse and morwong, with pink snapper, dhufish and samson fish on the deeper edges and western rock lobster in the crevices. The Marine Life Network notes that on the Quinns reefs the more interesting species tend to be found at depth.

Because the reefs are well offshore, visibility is usually better than at the inshore sites, but they are exposed and need calm weather. They are dived mostly from private boats; no operator runs regular trips. Published detail is limited to brief notes, so this record gives the position, depth and main features, and divers should check the reef on the sounder before descending. The depth means short no-decompression times, so Advanced certification and nitrox are recommended. Water temperatures range from about 17–18 °C in late winter to 22–23 °C in late summer.

Launch at the Mindarie Marina public ramp (Mindarie Keys, off Marmion Avenue, about 40 minutes from the Perth CBD). The reef is at S 31°37.580' E 115°36.600', a run of roughly 15–25 minutes north-west of the marina entrance depending on conditions. Use a sounder to find the reef edge and anchor in sand beside it rather than on the limestone.

Other charted reefs and the Conference, Two Fold Bay and Gemini wrecks are nearby, so two sites can be combined in one trip.

These reefs lie several kilometres offshore in open ocean with no island shelter. Dive in swell under about 1.5 m and light winds, ideally easterly. The south-westerly sea breeze usually arrives late morning to early afternoon in summer and makes the run home rough, so start early.

Some reefs in the group are occasionally affected by current. Winter fronts bring north-westerly gales and large swell. Visibility is generally better than inshore, often 10 m or more.

Anchor in sand beside the reef, descend to 15–19 m and follow the main reef feature (a 50 m wide ridge) in one direction, checking ledges and overhangs. Turn on half air and return along the reef top to the anchor, with a safety stop on the line.

Nitrox 28–32% extends bottom time.

Private boats from Mindarie; occasional Perth dive club and charter trips (Perth Scuba, Image Dive & Charters, UWA Underwater Club). No operator schedules this reef regularly.