Gary's Gut is the deepest site on the Agincourt Reef No. 4 roster, a terrific wall plunging from 5 to 40 m, according to Silverseries and Poseidon. The site notes suggest swimming out from the wall to take in the drop – advice that captures the dive's main appeal, which is scale. After the gardens and bommies elsewhere on Agincourt, Gary's Gut offers a great contrast.

The wall is steep and in places vertical, cut by gullies and overhangs. It is decorated with gorgonian fan corals and xenia soft corals that pulse gently in the current, as well as whip corals and sponges. The fish life includes unicornfish in schools off the wall, surf parrotfish on the reef crest, several species of butterflyfish, and whitetip reef sharks resting on ledges or cruising along the face.

The depth available means the dive must be planned carefully. Open water divers should stay at 18 m or shallower, which still gives a spectacular view down the wall into the blue, while advanced divers can go a little deeper to see the larger fans. Because the wall offers no natural floor, buoyancy control and depth discipline are essential, and guides monitor depths closely.

Visibility averages around 20 m and can reach 40 m, which lets you appreciate the full drop. The site is exposed to current at times, which feeds the fans and soft corals but can require some effort on the swim back to the mooring. Gary's Gut is ideal for Advanced Open Water divers or those wanting a taste of genuine outer-reef wall diving.

Photographers will find large fans for wide-angle compositions, and divers looking for the sense of hanging over the abyss will find it here.

Agincourt day boats leave Crystalbrook Superyacht Marina (formerly the Reef Marina) on Wharf Street, Port Douglas, between about 8:00 and 8:30 am. Park in the marina car park or the free all-day street parking around Wharf and Dixie streets; Quicksilver, Poseidon and Calypso also run coach pick-ups from Port Douglas, Palm Cove and northern Cairns beaches. The run to the Agincourt ribbon reefs on the edge of the continental shelf is about 65–70 km north-east and takes roughly 75–90 minutes on the large catamarans (Silversonic is the fastest), longer on small-group boats such as Scuba IQ/Blue Dive.

You gear up on board and giant-stride from the dive deck at a permanent mooring; the skipper picks the sites on the day to suit wind and tide, so a named site is never guaranteed. Boats are back at the marina around 4:30 pm.

Agincourt is an outer ribbon reef on the shelf edge, so it is fully exposed to the south-east trade winds that blow from about April to September. Moorings on the protected western (back-reef) side remain diveable in 15–20 knots, but anything over 25 knots or a 2.5 m+ sea makes the crossing rough and operators may cancel or switch to more sheltered moorings. Calm spells in October–December give the glassiest days and 30 m+ visibility.

The wet season (December–April) brings northerlies, afternoon storms, cyclone risk and occasional reduced visibility after big rain events, although the outer reef is far less affected by run-off than inshore reefs. Marine stingers are present November–May. Tidal currents run along the reef edges and through the gaps between the Agincourt reefs; spring tides strengthen them, so exposed corners are usually dived on the neaps or near slack water.

Descend the wall to your planned maximum (18 m for open water, up to 30 m for advanced divers), swim out a few metres from the wall to appreciate the scale, then follow it with the wall on one shoulder. Gradually ascend along the wall, finishing on the top at 5 m for the safety stop.

Keep to recreational limits; nitrox helps on repetitive days but check MOD – EANx32 MOD is about 34 m (ppO2 1.4).

Quicksilver Silversonic – daily, 3 dives across Agincourt 1–4; Poseidon (Quicksilver Group) – daily, 3 sites at Agincourt; Calypso Reef Cruises – regular Agincourt days; Scuba IQ / Blue Dive and Tech Dive Academy – small-group day trips several days a week.