Patersons Arch is a series of large swim-through arches in the sea cliffs between Tasman Arch and Waterfall Bay, a short boat ride from Pirates Bay. Descriptions of the site pair it with the Waterfall Bay caves and caverns as a dive accessible to all qualification levels, because the arches are open to daylight and there is always a clear exit ahead. The main arch is broad and high, framing the blue water beyond, and its ceiling and walls are coated with the shaded-habitat invertebrates typical of this coast: jewel anemones, golden zoanthids, sponges and bryozoans.

Schools of bullseyes hang in the shadows, draughtboard sharks rest on the floor and rock lobsters tuck into undercuts. Outside, the dolerite continues as walls and boulders with kelp in the shallows, where weedy seadragons are regularly seen. The combination makes Patersons Arch a good choice for divers who want an overhead-feeling experience without committing to Cathedral Cave's longer guided route, and it is a favourite with wide-angle photographers who can silhouette divers against the arch mouth.

The geology is the same that created the land features nearby – Tasman Arch, Devils Kitchen and the Blowhole – where the sea has eroded joints in the rock into caves that later collapse into arches and chasms. Depths range from about 6 m on the kelp-covered shelves to around 20 m at the base of the cliff. As with every cliff-base site here, the dive depends on the swell; on calm days it is easy and relaxed, while surge through the arch on rougher days makes it a site to skip.

Boat access only. Drive from Hobart on the Tasman Highway to Sorell, then the Arthur Highway through Dunalley to Eaglehawk Neck (about 75-80 km, 1 hour 15 minutes). Eaglehawk Dive Centre is on Pirates Bay Drive above the bay; divers kit up at the centre and the catamaran leaves from the Pirates Bay jetty/boat ramp at the northern end of the Neck (Old Jetty Road).

Go Dive Tasmania meets at The Adventure Lodge in Taranna and launches at Eaglehawk Neck or Port Arthur depending on conditions. Waterfall Bay is about 8 minutes south by boat from Pirates Bay, past Tasman Arch and Devils Kitchen; boats pick up a mooring or hover in the lee of the cliffs.

Waterfall Bay is the most sheltered part of the Tasman Peninsula's ocean coast and is the site operators fall back on when the Sisters and Hippolytes are blown out, because the bay faces east-north-east and the high cliffs block westerly and south-westerly winds. It is cancelled by easterly and north-easterly winds and by large south-easterly swell wrapping into the bay, which drives heavy surge into the caves and gutters. For cave and swim-through dives the swell needs to be small (well under about 1.5 m) or the surge becomes dangerous.

Heavy rain turns the waterfall on and can put a fresh, tannin-stained layer on the surface for a day or two. Late summer and autumn give the calmest, clearest conditions; winter mornings with westerlies are often excellent.

Descend at the base of the cliff beside the arch at 15-18 m, look through before entering to judge surge, then swim through the arch in the light zone. Explore the adjoining walls and gutters and return through or around the arch, finishing in the kelp at 5-6 m.

Eaglehawk Dive Centre – part of its Waterfall Bay caves and canyons rotation, often as the second dive; Go Dive Tasmania – Waterfall Bay area boat days.