Great Glennie Island is the big island of the Glennie Group, the cluster of granite islands that lies off Norman Bay in full view of the Tidal River campground, and the group is the offshore diving most Prom-based club trips actually do because it is only 5 km from the beach launch. The island is a steep granite dome with cliffs on its western face and boulder slopes on the east. Underwater the boulder slopes drop from bull kelp shallows through gullies and overhangs to sand at 25–35 m, with sponge, zoanthid, gorgonian and sea-whip cover increasing with depth, and the western faces are steeper walls.

Divers report seals regularly – the Glennies hold haul-outs – along with Port Jackson and draughtboard sharks, fiddler rays, blue devils, boarfish, morwong and schools of sweep and butterfly perch, and the kelp forests on the eastern side are among the healthiest on the Prom. Visibility is typically 15–25 m. The island's eastern side and Ramsbotham Rocks give shelter in south-easterlies, which is when the group is most often dived; the western walls need a rare flat day.

Great Glennie is the natural first offshore dive for a boat based at Tidal River, a step down in commitment from the Anser Group and Skull Rock but with the same granite scenery and clear water, and with Dannevig's cave and Bum Rock a few hundred metres to the south for a second dive. It is within the marine national park; no take.

Boat only. The nearest launch is the Tidal River beach ramp at the western end of Norman Bay (4WD beach launch, key and deposit from the Parks Victoria office; needs several people and calm surf), from which the western islands are 5–12 km. Larger trailer boats launch at Port Welshpool (sealed ramp, Corner Inlet) and run about 50–60 km around South East Point – 1.5–2 hours – or at Sandy Point (Waratah Bay, beach ramp) and run 35–45 km south-east.

Sites are exposed to Bass Strait and require settled weather, a seaworthy boat, radio, and self-sufficiency: there is no rescue or refuelling closer than Port Welshpool. The Glennie Group is the closest offshore target to the Tidal River beach launch – about 5–6 km west across Norman Bay, 15–25 minutes in a trailer boat – and it is the group most often dived by clubs camping at Tidal River. Anchor in the lee of the island in sand; Ramsbotham Rocks at the north-east tip gives shelter in south-easterlies.

Exposed to the prevailing south-westerly swell and wind on its western side, with usable shelter on the eastern, Prom-facing side and at the north-eastern tip in south-easterly winds. Needs swell under about 1.5 m and light winds; the Tidal River beach launch is itself surf-limited. Late summer and autumn; club trips report whole weekends lost to weather.

Anchor in the lee, descend to the boulder slope at 10–15 m and work along the island with the rock on one shoulder, dropping to 25–30 m on the steeper faces where the sponge cover begins, then return through the kelp gullies in the shallows where seals join divers. Ascend the anchor line with a safety stop; carry an SMB in case of drift.

No dive charter operates at Wilsons Promontory. Sites are dived from club and private trailer boats – Dive La Trobe, VSAG, Melbourne University Underwater Club and Dive Victoria Group run occasional camping-and-diving trips – launching at Tidal River (beach launch, permit/key from Parks Victoria), Port Welshpool or Sandy Point. Wilsons Promontory Cruises (Port Welshpool) runs sightseeing cruises to Skull Rock and the islands but no diving.