Cleft Island — better known as Skull Rock — is the imposing granite monolith off the southern end of Wilsons Promontory, named for the cleft cave that splits the rock face. Diving runs around the base out to about 30 m, with sheer walls plastered in sponges, gorgonians, ascidians and kelp. Marine life is dense: big crayfish, abalone, weedy seadragons, blue devils, schools of bullseyes, sweep and regular Australian fur seal encounters from the nearby colonies. Boat access only, long charter trips out of Port Welshpool or via liveaboard. Best in light northerlies with swell under a metre and tide near slack. Use the live 7-day wind and swell forecast on this page to spot a settled window. Advanced divers only — exposed Bass Strait water, strong currents around the rock, extremely remote, very long boat run each way, no margin for marginal forecasts.
How far you can see underwater — measured in metres. 10m+ is great, 5–10m is workable, under 3m is murk. Driven by wind, swell, and recent rain.
Long-period waves rolling in from the open ocean. Direction matters more than height — a S swell hits Portsea hard, but an E swell rolls past. Period over 12 s = real ocean punch.
Offshore (N or NE) flattens the surface and clears the water. Onshore (SE through SW) chops it up and stirs sand. Calm or light offshore is the magic combo.
This site faces the open ocean. The exposure caption above shows which directions slam in. Anything from the opposite side gets blocked — that's the safest window.
Slack water — the 30 minutes either side of high or low — is calmest and clearest. Mid-tide brings the most flow. Plan to be down at slack, up before the run picks up.
Bass Strait sits 14–16 °C autumn–winter, 17–19 °C summer. Below 16° a 7 mm hooded keeps you warm for 60 min+. Drysuit if you're going long.