Three Sisters is a pretty garden site on Agincourt Reef No. 4 built around three prominent bommies that sit about 50 m from the mooring. Silverseries and Poseidon note that it is often visited by juvenile grey reef whalers and that huge schools of fish congregate here. The three bommies rise from a sandy floor at up to 20 m to within a few metres of the surface, their walls covered in hard corals, sea fans and soft corals.

Each attracts its own congregation of fish: fusiliers stream around the upper parts, red bass and spangled emperor patrol the bases, and schools of snapper and sweetlips hang in the shelter of the coral. The fish density is the standout feature – on a good day the water between the sisters is thick with life. Grey reef whalers, mostly juveniles, cruise between the bommies and along the sand, and whitetip reef sharks rest on the bottom.

Stingrays lie in the sand channels, and turtles are regular visitors. Between and around the bommies is a garden reef of mixed hard corals, which is where snorkellers spend their time. The dive plan is simple and satisfying: swim out to the bommies, visit each in turn and return over the garden reef.

The distance from the mooring is short enough that no one gets lost, and the bommies make obvious landmarks. The site suits all levels, although divers should keep an eye on depth when circling the base of each sister. Visibility averages around 18 m and often reaches 30 m.

Three Sisters is a reliable choice for a relaxed but fishy dive, and it is an excellent place for photographers to capture schools of fish framed by coral pinnacles. Do not confuse it with the Three Sisters at Michaelmas Reef, a Cairns site visited by Silverswift.

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.

Swim the 50 m from the mooring to the first bommie, then visit all three in turn, circling each from 15–18 m to the top. Watch the sand between them for stingrays and whalers, and return to the mooring over the garden reef at 6–8 m for the safety stop.

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.