Seal Rocks is a cluster of low granite and basalt islets 1.5 km off The Nobbies at the western tip of Phillip Island, and it holds the largest colony of Australian fur seals anywhere – around 25,000 to 30,000 animals at the summer peak, roughly a quarter of the species. The rocks are a State wildlife reserve with strict approach rules, and the underwater experience is defined by the seals: in the water they are fast, curious and fearless, and a diver holding still in a kelp gully at 10 m will be circled, nudged and stared at by a succession of juveniles and females, with the big bulls generally staying on the rocks. The topography is a boulder and kelp slope dropping from the islets to sand at about 20 m, cut by surge gullies on the exposed side and steeper on the tidal-channel side, with dense bull kelp on the shallow rock and sponge, ascidian and bryozoan growth in the lee under the ledges.

Fish life is good and characteristic of the Western Port entrance – schools of sweep and salmon in the tide, wrasse, morwong, leatherjackets and old wives on the reef, crayfish in the deeper cracks, and the occasional Port Jackson shark – but nobody comes here for fish. The rocks are dived far less than they are snorkelled. Ocean Adventures Phillip Island and other Cowes operators run seal-swim trips through the warmer months, keeping swimmers on the surface in the lee of the rocks, while scuba divers arrive on private or club boats and need slack water in the strong tidal race that runs between the islets and Point Grant.

Great white sharks are recorded here, drawn by the colony, and most divers accept that as part of the site. For Victorians, Seal Rocks is the state's best big-animal encounter and the equal of Montague Island or the Neptunes for sheer numbers; for a diver with a boat, a calm day and the right tide, it is one of the most memorable shallow dives on the coast. The approach distances under the Wildlife (Marine Mammals) Regulations are enforced and apply to boats, snorkellers and divers alike.

Boat only. Snorkel tours leave from the Cowes Jetty area (Ocean Adventures Phillip Island and Wildlife Coast Cruises run seal cruises; only some offer in-water swims) and take 20–30 minutes to reach the rocks. Private boats launch at Cowes (Mussel Rocks ramp) or Rhyll, about 12–15 km and 25–35 minutes west along the north coast and around Point Grant; Newhaven is a longer run around the south coast.

Approach the rocks on the lee side, keep the boat live, and observe the Wildlife Act approach distances – vessels must stay at least 50 m from the rocks' seal haul-out (and further during breeding season) unless permitted; people in the water may not approach within 30 m of a seal on land or 5 m of seals in the water. Check Phillip Island Nature Parks and DEECA for current rules.

The rocks sit in the tidal race at the entrance to Western Port, so streams of several knots run past them on both flood and ebb; snorkel operators work the lee side at or near slack and let swimmers drift with the flow, and scuba divers should plan for slack water and carry an SMB. Exposed to south-westerly and westerly swell – anything over 1.5 m makes the site untenable and washes across the low rocks. Best in light easterly or northerly winds.

Operators run mainly October to May; seal numbers peak in summer after the November–December pupping season. Great white sharks are recorded in the area, particularly in autumn and winter.

From a live boat on the lee side at slack water, enter and drop to the kelp-covered rock slope at 8–15 m, then hold position in a gully or drift slowly along the islets with the current, letting the seals come to you rather than approaching them. Turn or signal the boat at half gas or when the stream picks up, ascend under an SMB in open water clear of the rocks, and be collected by the boat. Snorkellers stay on the surface within the operator's marked area.

Ocean Adventures Phillip Island (Cowes) – guided seal-swim/snorkel trips several times a week in season; Wildlife Coast Cruises (Cowes) – seal-watching cruises (viewing, not diving). Scuba is by private or club boat only; no dive charter runs Seal Rocks regularly.