Boulders is the site on Flynn Reef where divers are most likely to feel current, and that current is what makes it worth doing. The dive follows a wall that drops from a shallow reef top to around 25 m, and the flow along it keeps the coral fed and the fish active. Dive the World singles Boulders out as Flynn's current wall, with shallow exploration and giant clams up top, and Cairns operators list it among the nine named Flynn moorings.

The wall face is covered in hard corals with soft corals and sea fans on the more exposed sections, and the shallows are scattered with large boulder corals, which presumably gave the site its name, and giant clams. Fish life is typical of a current-swept outer reef: schools of fusiliers and surgeonfish in the water column, trevally and reef sharks along the wall, oriental sweetlips and groupers sheltering in the undercuts, and parrotfish and wrasse working the top. Turtles are common on Flynn and often seen here.

The wall gives an easy reference and the current does most of the work if the dive is run as a drift, so it is a good introduction to current diving for divers who already have a few logged dives. On a strong spring tide it is better left for another day. The shallow reef top gives a sheltered finish, and a long safety stop among the clams is a pleasant way to end.

Photographers should be ready for fast-moving subjects on the wall and for large clams and boulder corals in the shallows.

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 57 km out through Trinity Inlet and across the lagoon to Flynn Reef takes around 80 minutes on a fast catamaran and noticeably longer on slower dive boats or into a south-easterly chop.

Boulders is one of the Flynn moorings named by Silverswift and Cairns Dive Adventures and used by day boats and liveaboards when conditions suit a current dive. Enter from the stern and descend promptly.

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. Boulders carries current along the wall and is normally dived on the neap tides or around slack water; guides may run it as a gentle drift with the boat picking up.

Descend quickly to the wall, check current direction, and swim into it (or drift with it if the guide plans a pick-up) at 15-20 m. After 20-25 minutes rise onto the shallow reef top where the giant clams sit and finish the dive there out of the current, with a safety stop at 5 m.

Silverswift – occasional Flynn day-trip site; Pro Dive Cairns – liveaboard; Flynn is also worked by Silverswift, Passions of Paradise and the Pro Dive Cairns liveaboard; the mooring used on any day is chosen by the skipper according to wind, tide and other boats