St Crispin is an elongated reef on the outer shelf, lying south of the Agincourt reefs and east of Undine Reef. Rather than the typical outer-reef slope, its structure consists of many shallow sandy channels that reticulate around coral mounds, according to its Wikipedia entry, and it sits in water around 9 m deep over much of its top. Protected by neighbouring reefs, it is one of the gentler outer reefs to dive and snorkel out of Port Douglas.

The reef is nicknamed the Flower Garden for its soft corals and colour, and the southern end (see the separate Flower Gardens entry) is the best-known site. Wavelength, which holds two of its four moorings, describes large schools of fusiliers and sergeant majors, friendly sweetlips, feather stars, bumphead parrotfish, wrasse and butterflyfish, with occasional shy reef sharks. Smaller life includes brightly coloured nudibranchs, sea squirts, urchins and hermit crabs, and the sandy zones hold sea cucumbers and giant clams.

The maze of channels and mounds gives the dive an intimate feel – you wander between coral heads rather than following a wall – and the shallow depth means long bottom times and bright colours. It is a relaxed site for new divers and a very good snorkel reef. St Crispin also has a sombre place in diving history.

In January 1998 American divers Tom and Eileen Lonergan were accidentally left behind by their dive boat here and never found; the case led to major changes in Queensland's head-count regulations and inspired the film Open Water. GBRMPA classifies the reef for intensive use, but with only four moorings it rarely feels busy. For divers wanting an easy, colourful outer-reef day away from the larger pontoon operations, St Crispin is a good option.

St Crispin trips leave Crystalbrook Superyacht Marina, Wharf Street, Port Douglas, around 8:00–9:00 am (Wavelength departs 8:00 am and returns about 4:30 pm; Calypso 9:00 am). Park at the marina or on nearby streets, or take the operators' pick-up. St Crispin is an outer-shelf reef about 55 km north-east of Port Douglas, south of the Agincourt reefs and east of Undine Reef; the run takes roughly 75–90 minutes.

There are only four public moorings on the reef (Wavelength holds two), and divers and snorkellers enter from the boat's stern at the mooring.

St Crispin is an elongated outer-shelf reef protected by neighbouring reefs, which gives it some shelter from the south-east trades that blow from about April to September, so several moorings stay diveable in 15–20 knots, but a 25-knot-plus trade wind makes the crossing rough and operators may cancel. Exposed sites on the seaward face and in the channels at the southern gap are dived only in light winds and suitable tides. October–December is typically calmest with the best visibility; the wet season (December–April) brings northerlies, storms, cyclone risk and marine stingers (November–May).

Tidal currents run through the gaps between the Opal reef sections and around bommies, strongest on spring tides – channel and bommie sites are timed for neaps or slack water.

From the mooring, follow one of the sandy channels between coral mounds at 6–10 m, looping around a mound or two and returning along a parallel channel. Keep track of the mooring, since the channels look alike, and do the safety stop on a mound top near the boat.

Wavelength Reef Cruises – snorkel-only trips several days a week (holds two of the four moorings); Calypso Reef Cruises – occasional dive/snorkel days; small-group dive charters on request.