Granite Island is Victor Harbor's best-known landmark, a rounded granite island linked to the town by a causeway traversed by a horse-drawn tram. The Ramindjeri knew it as Nulcoowarra. It was a whaling lookout in the 1830s, and in the 1860s-1870s the jetty, breakwater and causeway were built when Victor Harbor was a major port for Murray River trade; commercial importance declined after 1914.

Today the island is a recreation park with a cafe, walking trails and a much-reduced little penguin colony, which fell from about 1,800 birds in 1998 to only a couple of dozen in recent counts. The Marine Life Network lists Granite Island as a dive site to about 15 m, noting blue devils, boarfish, perch and sweep among the granite boulders on the southern side. The boulders continue underwater as a jumbled slope with gaps and overhangs, covered in kelp on top and sponges and invertebrates beneath.

Seals and fur seals are common around the island, and the Big Duck and other boats take visitors past. Detailed published dive guides for the island are scarce, so divers should treat this as an exploratory boulder dive. The southern side is exposed to the Southern Ocean swell and needs genuinely calm conditions; the northern, causeway side is more sheltered but shallower and less interesting (see Granite Island Jetty & Breakwater).

The site suits experienced shore or boat divers on northerly days. Be mindful of the penguin colony and respect Recreation Park rules ashore. Carry a surface marker and let someone ashore know your plan, as the southern side is out of sight of the causeway.

From the Victor Harbor foreshore, walk across the causeway (about 600 m) to Granite Island – the horse-drawn tram also runs across for visitors, but carrying scuba gear is easier on a trolley on foot. The boulder dives are on the southern side of the island, reached by the island's walking paths; entry is over granite boulders, so choose a sheltered spot. Boats can launch at the Bluff ramp and run about 10 minutes to the southern side.

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.

The southern side of Granite Island faces the open swell directly.

From a sheltered entry or anchored boat on the southern side, descend along the boulders to 10-15 m and follow the slope with the island on one shoulder, looking into gaps between boulders. Turn at half gas and return shallower.

No regular operator; private boats and occasional club dives.