Blue Lagoon on the Agincourt complex is, according to Scuba IQ, largely inaccessible because of its tricky layout, and a rare treat that requires perfect conditions. It is not a site you can book; it is one the guides take divers to when everything lines up. Lagoon sites on the outer ribbon reefs are enclosed or semi-enclosed areas behind the reef crest where coral grows in sheltered water.

Access is the challenge: moorings may be limited, entry channels shallow, and the tide and wind must allow a boat to reach them safely. When conditions permit, divers find calm, clear water, sandy floors and coral bommies growing up to the surface. Scuba IQ gives no further detail on the site's features or depths.

Expect shallow water, sheltered conditions and a high density of coral and reef fish – the reward for the effort of getting there. Guides will brief the specific route and turnaround time. Visibility inside lagoons can be excellent on calm days, although it may reduce with tidal flow.

Because conditions are critical, the dive may be cancelled or changed at short notice. Blue Lagoon suits divers who have done the regular Agincourt sites and want something unusual. It should not be confused with the Blue Lagoon on Opal Reef or at Lizard Island.

If you are diving several days with a small-group operator such as Scuba IQ or Blue Dive, it is worth asking whether conditions might allow a visit – light winds, a neap tide and a boat small enough to reach the mooring are usually the prerequisites. Because the name is operator-specific, other boats may know the same area by a different name or not use it at all.

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.

Only run in calm, favourable tides. The guide leads the group into the lagoon along the safest route; stay close, keep shallow (typically under 12 m) and follow the guide back out the same way before the tide changes.

Scuba IQ / Blue Dive – small-group Agincourt day trips (max ~20 guests) several days a week; the site names are Scuba IQ's own and may not be used by the larger Quicksilver/Poseidon boats.