Harry's Bommie on Agincourt Reef No. 4 is described by Silverseries and Poseidon as a stunning garden reef scattered around pinnacles and patch reefs, and a favourite among dive instructors. That instructor endorsement says a lot: it is a site that delivers consistently, in easy conditions, with enough variety to keep experienced divers interested. The layout is a garden of coral pinnacles and smaller patch reefs spread over a sandy floor with a maximum depth of around 16 m.

Each pinnacle is its own mini-ecosystem covered in hard and soft corals, and the sand channels between them make navigation straightforward – you move from bommie to bommie, circling each before crossing to the next. The site is regularly visited by juvenile grey reef whalers, which patrol the edges of the garden, and manta rays are seen occasionally, particularly in the cooler months. Groper and oceanic sweetlips hang beneath overhangs, oceanic trout (coral trout) sit on coral heads, and moray eels peer from holes.

Anthias swarm in orange clouds above every outcrop, and rays rest on the sand. Because it is shallow and sheltered, Harry's Bommie suits all levels, including discover-scuba and newly certified divers, and it is used for training dives. Yet the regular appearance of young sharks and the chance of mantas mean seasoned divers enjoy it too.

Visibility averages about 15 m and ranges up to 30 m. The combination of depth, light and structure makes it an excellent photography site – schooling anthias against pinnacles, or a young whaler cruising past a coral garden. It is part of the regular rotation for Silversonic and Poseidon, and the relatively short distances between features mean you can cover the whole site in one relaxed dive without feeling rushed.

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.

From the mooring, tour the cluster of pinnacles at 10–16 m, crossing the sand between them and watching the edges for juvenile grey reef whalers and rays. Spend the second half of the dive in the garden reef at 6–10 m and do the safety stop on the top of the main bommie.

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.