Seal Island, also known as Seal Rock, is a small granite outcrop in Encounter Bay about 2.7 km south-east of Victor Harbor and 3.7 km east-north-east of Rosetta Head. Wikipedia describes it as a one-hectare mound of tumbled granite boulders scoured of soil, rising 12 m, with no vegetation and, at last survey, a breeding colony of silver gulls. It has been part of West Island Conservation Park since 1979, and its waters are within Encounter Marine Park.

The Marine Life Network lists 'Seal Rocks' as a boat dive to 25 m on huge granite boulders, covered with sponges, gorgonians, zoanthids, bryozoans and ascidians, with morwong, Port Jackson sharks, yellowtail and kingfish. That matches the island's geology, and the site is taken here to be the same place. The boulders create a three-dimensional landscape of gaps, overhangs and walls, and the invertebrate growth can be colourful in good light.

The site requires good boating weather and is exposed to swell from the south, so it is dived only occasionally by locals with their own boats. Currents can run around the island on big tides. Despite the name, seals are not documented as resident, but they are common throughout Encounter Bay and may visit.

Seal Island suits Advanced divers comfortable with boat diving in open water, looking for a deeper, less-visited granite boulder dive than the Bluff. Combine it with Granite Island or Wright Island for a varied boat day out of the Bluff ramp. Watch no-decompression limits closely if descending to the base of the boulders, and plan a gradual ascent up the granite.

Boat access only. Launch at the Bluff boat ramp (about 15 minutes' run east past Granite Island) or, for small boats, from the Victor Harbor area. The Marine Life Network notes the site requires good boating weather.

Anchor in sand beside the boulders; do not land on the island, which is part of West Island Conservation Park. Fly a dive flag throughout, as tour boats and fishing boats pass the island regularly.

Encounter Bay faces south into the Southern Ocean swell that wraps around Kangaroo Island and Cape Jervis, so Victor Harbor sites are the reverse of the gulf coast: they need northerly (offshore) winds. The best days follow several days of northerlies with low southerly swell, when visibility improves and the swell is blown back; any southerly or south-westerly swell over about 1 m, or strong southerly winds, makes most sites undiveable and stirs up particles. Summer and autumn bring the most northerly days, but good windows also occur in winter ahead of cold fronts.

Tidal currents strengthen around the headlands and islands during large tidal movements. Southern right whales visit Encounter Bay from about June to October – keep legal approach distances. Water is a little cooler than in the gulf.

Seal Island is in the open bay and needs a calm, northerly day.

Anchor off the boulders, descend to 20-25 m at their base and work upward around the island, exploring gaps and overhangs for Port Jackson sharks, then finish on the shallower boulders.

No regular operator; private boats only. Wildlife boat tours (e.g. The Big Duck) visit for sightseeing, not diving.