The Gap is a lagoon-type gutter that cuts in close to the crest of Agincourt Reef No. 3. It is shallow – nowhere much deeper than 12 m – and it is dived as a gentle drift and snorkelled from the surface. The Silverseries and Poseidon site notes describe a fascinating reef wall with numerous small grottos, backed by boulder and staghorn coral beds.

The gutter works like a natural funnel: as the tide fills and drains the back-reef lagoon, water moves along it, carrying plankton and keeping the coral healthy. One side is a low reef wall pocked with small caves, overhangs and grottos. Look inside and you will find squirrelfish, cardinalfish, cleaner shrimps and sometimes a sleeping whitetip reef shark or a crayfish.

Out in the gutter itself, golden goatfish work the sand in bright yellow groups, and large schools of sweetlips hang motionless in the shelter of coral bommies during the day. Groper – big Queensland groper or estuary cod – are often seen lurking under ledges. The flanking beds of boulder and staghorn coral are home to a mass of damselfish, chromis and wrasse, and the shallow depth means the colour is superb in natural light.

Because it is close to the crest, The Gap is a site for calm days, when the trade winds are light and the tide is manageable. The drift is gentle, and newer divers can enjoy it as long as they stay with the guide; snorkellers get an equally good view of the grottos and fish from above. This is a relaxed, low-depth dive that makes a good final dive of the day, reducing nitrogen load while still delivering plenty of fish and interesting reef structure.

It suits photographers looking for sweetlips schools and anyone who likes poking around caves without committing to deeper swim-throughs.

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 at the head of the gutter with the guide, settle at 6–10 m beside the reef wall and drift slowly along it, peering into the grottos. Keep your buddy close and stay near the SMB; ascend together at the end of the gutter for pick-up, doing your safety stop at 5 m along the wall.

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.