Horseshoe Reef is the northern edge of Agincourt Reef No. 2 run as a drift dive, and it is one of the more exhilarating dives on the Port Douglas day-boat roster. The Silverseries and Poseidon site notes single out one image: huge schools of luminescent blue fusiliers hugging the reef wall. On a good tide you fly past a continuous curtain of them.

The reef edge drops from about 5 m down to around 20 m, with a mix of hard coral buttresses, overhangs and soft coral growth. The current funnelling along the edge carries plankton, which is why the fusiliers are there in such numbers, and why the predators follow. Trevally and Spanish mackerel make fast passes through the schools, red bass and humphead Maori wrasse patrol the wall, and big humphead parrotfish move along the reef top in herds.

Keep watching the blue side as well as the wall: reef sharks and eagle rays are regular sightings on this kind of edge. As a drift, the dive involves very little finning – you position yourself a few metres off the wall, match depth with your buddy and let the water do the work. The guide carries a surface marker buoy and the boat's tender tracks the group and collects everyone at the end.

The main skills required are good buoyancy control and staying close to the group, which is why it is best for divers with a few dives under their belt. On slack or neap tides the site can instead be dived from the mooring as an out-and-back along the wall. Visibility commonly runs 20 m or more.

Horseshoe Reef suits confident open water divers and anyone who enjoys the sensation of drift diving. It offers a big-fish, big-school experience that contrasts nicely with the gentler coral gardens elsewhere on Agincourt 2.

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.

Drop in as a group up-current of the reef edge, descend quickly to 12–15 m against the wall and let the current carry you along the northern edge with the reef on one shoulder. Stay close to the guide's SMB, move up gradually to 8 m in the last third of the dive and ascend together for a tender pick-up, doing the safety stop mid-water.

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.