Phil's Lagoon is an unusual formation on the southern side of Third Sister reef off Tongue Reef, visited by Wavelength Reef Cruises. The operator describes a virtually enclosed lagoon full of coral and reef fish, with an exterior drop-off where grey reef sharks pass and schools of plankton-feeding fusiliers gather. The enclosed lagoon is the main attraction.

Protected from waves, it supports delicate coral growth and a dense community of reef fish. Enormous giant clams are a feature, and turtles are frequently seen. The calm water makes it ideal for snorkellers of all abilities.

Outside the lagoon, the reef drops away, providing a contrast. Grey reef sharks cruise along the drop-off, and schools of fusiliers feed on plankton in the water column. Snorkellers can look over the edge into deeper water, and divers on a charter would find a wall to explore.

No depth figures are published; the lagoon is shallow, and the drop-off extends into deeper water. The site should not be confused with Phil's Reef or Phil's Backside on the Agincourt complex. The lagoon is sheltered enough to be used on days when the more exposed corner sites are too lively.

Wavelength is a snorkel-only operator whose marine biologists guide in-water tours and give reef talks; its skipper chooses three sites each day from the company's exclusive moorings according to wind and tide, so a particular site is never guaranteed. Scuba divers wanting to dive the Sister reefs need a private charter, and because the site names are Wavelength's own, other operators may not recognise them.

Wavelength Reef Cruises departs Crystalbrook Superyacht Marina, Wharf Street, Port Douglas, at 8:00 am and returns about 4:30 pm; park at the marina or on surrounding streets. The Sister reefs lie just off the seaward side of Tongue Reef and the crossing takes about 90 minutes, threading through the Tongue Reef maze. Wavelength is snorkel-only and holds exclusive moorings here; snorkellers enter from the stern platform at the mooring.

Scuba access requires a private charter with a small-group dive operator.

The Sister reefs sit on the seaward side of Tongue Reef close to the shelf edge and are exposed to the south-east trades that blow from about April to September, so moorings are chosen to suit the wind and large swells rule out the exposed sites; a 25-knot-plus trade wind makes the crossing rough and operators may cancel. Exposed sites on the seaward face and in the channels through the major reef gaps 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.

Snorkel inside the enclosed lagoon among the coral and giant clams, then, if conditions allow, follow the guide to the outer drop-off to look for grey reef sharks and fusilier schools.

Wavelength Reef Cruises – snorkel-only full-day trips with exclusive (low-impact) moorings at Third and Fourth Sister; scuba only by private charter (Scuba IQ / Blue Dive, Tech Dive Academy).