Meet the site
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.
What you might see
Golden goatfish, schooling sweetlips (diagonal-banded, ribbon), Queensland groper, squirrelfish, cardinalfish, cleaner shrimps, whitetip reef sharks, damselfish, boulder and staghorn corals
Location
A lagoon-type gutter close to the reef crest of Agincourt Reef No. 3, dived as a shallow drift and snorkel.
Facilities nearby
Departs Crystalbrook Superyacht Marina, Wharf Street, Port Douglas – car parking, toilets, cafes and restaurants at the marina; dive shops in town for gear hire and air
Before you make plans
Conditions, access and operator schedules can change. This is a location guide, not a dive plan. Ask a local operator about current conditions and suitability for your training and experience.
Sources & further reading
silverseries.com.auposeidon-cruises.com.auRecord updated 26 Sept 2026. Source claims are being reviewed.