Blue Lagoon on Hastings Reef is a Divers Den site on the southern side of the reef, and it is quite different from the similarly named lagoon on Thetford Reef. Divers Den describes a shallow shelf of coral with a deeper shelf below it and walls extending on either side, so the dive is a pleasant combination of terraces and walls rather than a sandy lagoon. The shallow shelf is bright and colourful, well suited to snorkellers and new divers, while the lower shelf and walls give certified divers depth and structure.

A feature of the site is elephant-ear coral, the broad, folded colonies of Mycedium, with small shrimps hiding in the folds, surrounded by table corals. Hastings Reef as a whole supports butterflyfish, damselfish, clams, turtles and small reef sharks, according to Divers Den, and Cairns Dive Adventures adds large schools of humphead parrotfish, which move around the reef in the mornings. On the walls look for anemonefish, angelfish, coral trout and cod, and on the terraces for giant clams and sea stars.

The dive is straightforward and suits all levels; less experienced divers can stay on the upper shelf and still see plenty. Blue Lagoon and Angel Bommie are close enough that guides sometimes combine them in one dive. Angel Bommie, a sandy staghorn bay with its own entry, adjoins it, and the two are close enough that guides sometimes combine them in one dive.

Visibility is usually 12-20 m, and the upper shelf is at its most colourful on a sunny day around high tide.

Boat access only. Cairns day boats leave from the Reef Fleet Terminal (1 Spence Street) and the neighbouring Marlin Marina on the city waterfront, with check-in between about 7:00 and 8:00 am; most operators run transfers from city and northern-beaches accommodation, and there is paid parking near the terminal. The run of about 50 km out through Trinity Inlet and across the lagoon to Hastings Reef takes around 75 minutes on a fast catamaran and noticeably longer on slower dive boats or into a south-easterly chop.

Blue Lagoon is used by Divers Den's Reef Quest day boat and Ocean Quest liveaboard and listed by Cairns Dive Adventures among Hastings' moorings. Enter from the stern platform.

The prevailing south-east trade winds (typically 15-25 knots from April to September) are what limit Cairns outer-reef diving: above about 25 knots trips may be cancelled, and exposed eastern moorings are used only in light winds. Summer northerlies are gentler but make north-facing moorings choppy. Box jellyfish and Irukandji are present from about November to May (stinger suits on every dive), and the cyclone season runs November to April, when a system in the Coral Sea can close the reef for days.

Visibility drops after heavy rain and strong winds; spring is usually calmest and clearest. Hastings has moorings on all sides, so there is almost always a sheltered option. Blue Lagoon is on the southern side of the reef and is best in northerly or light winds; in a fresh south-easterly operators usually move to the northern moorings.

Descend onto the shallow shelf, drop over its edge to the deeper shelf at 12-18 m, and follow one of the walls with the reef on your shoulder, checking the elephant-ear corals for shrimp. Return along the upper shelf and do the safety stop on the coral at 5 m.

Divers Den (Reef Quest day trips, Ocean Quest liveaboard) – regular; other Hastings operators occasionally; Hastings is also worked by Reef Experience, Divers Den, Tusa Reef Tours, Aqua Quest, Down Under Cruise & Dive and the Reef Encounter liveaboard; the mooring used on any day is chosen by the skipper according to wind, tide and other boats