The Arch is a compact but photogenic boulder site just offshore of Moon Island at the mouth of Swansea Channel, built around two large granite boulders, one of which is pierced by a genuine swim-through archway roughly 10 m wide and 3-4 m high. The seabed here is sand scattered with smaller rocks, and the arch itself sits with its northern entrance in about 17 m, while the second boulder rises to within 4-5 m of the surface from a base around 20 m. It's a straightforward navigational dive: drop on the eastern side, work along the reef to the arch, swim through, and loop back around the second boulder — a route that can comfortably be repeated slowly for photography.The site's main drawcard for underwater photographers is the light: with the sun in the right position the archway frames a classic sunburst shot, and the fixed life on the boulders — sea squirts, gorgonian fans, sponges, ascidians and a healthy population of crinoids — provides plenty of foreground interest.

Fish life includes large schools of yellowtail and seapike that hold in the lee of the boulders, and at least one very large resident cuttlefish is regularly seen inside the arch itself. Because it's a short, sheltered run from the Swansea boat ramp, The Arch works well as a second dive on a day trip alongside the deeper Swansea-area wrecks, or as a relaxed macro and wide-angle dive in its own right. It isn't heavily promoted or frequently dived compared with the Terrigal reefs further north, which keeps it a quieter option for club groups and private boats out of Swansea.

Launch from the Swansea boat ramp or via Swansea Channel and run a few minutes out to Moon Island; Aqua Zero (Swansea) and Norah Head Divers Depot both service the site by boat. No shore access — the arch sits offshore of the island in open water.

Exposed to swell around Moon Island; best attempted in under 1 m swell with light winds. Visibility is clearest after several calm days and drops after any swell event stirs the sandy bottom around the boulders.

Anchor to the east of the boulder group and follow the reef west to the northern entrance of the arch, at about 17 m; swim through east-west, then circle the western side of the boulder back to the anchor, keeping an eye out for the resident large cuttlefish inside the arch.

No scheduled charter; dived by Aqua Zero (Swansea) and Norah Head Divers Depot on request, and by private boats out of Swansea.