Jack's Patch is a lesser-known south-coast reef that rewards divers with their own boat. Marinelife.org.au describes it as a north-south strip of limestone reef with hard and soft corals, swim-throughs and ledges, and one large cave at its northern end. The shallowest point is about 13 m, most of the reef is at 15-17 m, and the southern and western sections drop deeper, to about 27 m.

The coral cover reflects the influence of the Leeuwin Current along Rottnest's south coast, and the ledges and swim-throughs provide good macro photography, according to the guide. It also notes that there is limited crayfish habitat, so this is a dive for reef scenery and small life rather than lobster. Expect buffalo bream, western king wrasse, bullseyes, morwong and sweep, with the chance of larger fish in the deeper sections.

The site's appeal is its combination of moderate depth, reef structure and relative obscurity: it is not a standard charter site, so it tends to be unspoilt. The trade-off is that it is exposed to the south-west and is best on a calm day with excellent visibility. It suits Advanced divers or experienced Open Water divers who can manage 15-20 m on air and navigate a linear reef. Pair it with the shallow Parker Point drop-off or Salmon Bay reef for a second dive.

As with other boat-only reefs on the south coast, carry a DSMB, fly a flag and keep someone on the boat in case the anchor drags in the afternoon sea breeze.

Launch at Fremantle Fishing Boat Harbour or Woodman Point and run around the east end of Rottnest to the south coast (about 20-25 km, 45-70 minutes in a trailer boat). Locate the site by GPS and echo sounder and anchor in sand clear of the reef, or live-boat. Jack's Patch lies roughly half a nautical mile south-west of Salmon Point; use GPS and the sounder to find the reef rising to about 13 m.

South-coast site exposed to southerly and south-westerly swell and sea breeze; dive in northerly or north-easterly winds with low swell. Marinelife.org.au notes it is great when visibility is excellent - usually summer mornings and calm winter days between fronts. General pattern at Rottnest: summer brings calm morning easterlies followed by a strong south-westerly sea breeze from late morning, so dive early; winter brings north-westerly fronts and large swell, with clear, calm spells between them.

Check the Bureau of Meteorology coastal forecast for swell height and period before committing.

Descend onto the shallowest part (about 13 m), follow the strip north to the large cave at its northern end, explore the ledges and swim-throughs at 15-17 m, and visit the deeper south and west sections only if gas and no-deco limits allow.

No regular operator - private boats and dive clubs (e.g. UWA Underwater Club) only. Nearest dive services: Perth Scuba, Bucket List Diver and Dolphin Scuba (Perth/Fremantle) run Rottnest boat trips; there are no dive shops or air fills on the island.