Monument Bay sits beside Monument Rock on the Cape Hauy side of Fortescue Bay's southern headland. The Marine Life Network guide lists it at 10-25 m, with a sloping reef, caves and excellent visibility, and notes crayfish and plenty of macro-photography subjects. The sloping reef makes this an easier, more relaxed dive than the current-swept Monument outside, and the caves along the reef provide shaded habitat for jewel anemones, sponges and other invertebrates.

Rock lobsters shelter in the cave entrances and cracks, and small subjects such as nudibranchs, sea spiders, gobies and blennies reward macro photographers who take their time. The shallower reef is covered in kelp and brown algae with wrasse, leatherjackets and magpie morwong. Clean water sweeping in from the Tasman Sea gives the bay the excellent visibility the guide notes.

It is a good second dive for private boats working the Cape Hauy area, and a suitable site for Open Water divers with some cold-water experience. The scenery above water – the dolerite columns of Cape Hauy and the Lanterns – is spectacular on the run in. Information is limited to the older guides, so treat depths and features as indicative.

The bay's position also makes it a practical refuge: when current is running too hard at the Monument or swell is building on the outer rocks, a boat can often tuck in here for a gentler dive before returning to the Fortescue Bay ramp. Take a torch for the caves and a macro lens for the small life on the walls.

Boat access only. Launch at the Fortescue Bay boat ramp (Fortescue Road, 12 km gravel from the Arthur Highway near Taranna; park pass required) and run around Cape Hauy, about 10 minutes. Charters from Eaglehawk Neck take about 30 minutes.

Anchor in sand clear of the reef in the lee of the headland. The Fortescue Bay ramp is concrete with a short jetty and car/trailer parking by the campground; there are no fuel or supplies at the bay, so fill up in Taranna or Nubeena.

The Tasman Peninsula faces the full Southern Ocean and Tasman Sea. South-westerly to south-easterly groundswell and strong easterlies are what cancel trips: operators generally need westerly or north-westerly winds with swell under about 1.5-2 m for the open-coast sites. Prolonged southerly swell pushes surge deep into the caves and gutters and churns visibility; after a big blow allow two or three days to settle.

Late summer and autumn (February-May) bring the most settled windows and the clearest water; winter can be glassy between fronts but fronts arrive every few days. Monument Bay is somewhat sheltered by Cape Hauy from north-easterlies but is open to southerly and south-easterly swell.

Descend on the reef at 20-25 m, then work up the slope into the caves at mid-depth with a torch. Finish in the shallows at the back of the bay with a slow safety stop.

No regular scheduled trips; private boats from Fortescue Bay and occasional club visits.