Barrel Rock is a rock in the Tamar entrance marked by a beacon because it obstructs shipping. The Marine Life Network guide rates it 7 out of 10 as an alternative to Hebe Reef, with 25-30 m depths and filter feeders dominated by thick, colourful sponge growth, plus fish life.

The guide warns that it is a slack-water-only dive in the main shipping channel, that noise from passing ships can be heard under water, and that divers need to monitor depth. It is only for experienced divers in close contact with Port Control. As with all Tamar entrance sites, local knowledge of the tides is worth more than any written description, and a first visit with Launceston club divers is the best introduction.

The Tamar entrance is one of the most tidally energetic dive areas in Tasmania, and the Marine Life Network guide classes several sites here as category 4-5 because of current and shipping. Hebe Reef, a shallow reef at the mouth, has claimed many ships since the early 19th century, and its wrecks are protected historic sites. The flip side of the current is prolific filter-feeding life - sponges, bryozoans and ascidians - on any structure the water flows over.

Launceston divers have access to these sites within an hour of the city, and the sheltered Low Head shore dive is a regular training and club venue. The nearest hyperbaric chamber, however, is at the Royal Hobart Hospital about 3 hours' drive south, as Launceston General Hospital has none, so plan slack-water dives conservatively.

Low Head is at the eastern side of the Tamar River mouth, 5 km north of George Town and about 55 minutes (55 km) north of Launceston via the East Tamar Highway (A8). Boats launch at the Low Head, George Town or Kelso ramps. The Tamar is a commercial port (Bell Bay), so contact Port Control on VHF and keep out of the shipping channel when ships are moving.

Barrel Rock is in the main shipping channel near Low Head, a few minutes by boat. Check with Port Control and dive only at slack with no ships moving.

The Tamar heads are dominated by tide: large volumes of water flood and ebb through the entrance, and most boat sites can only be dived at slack water, typically within about an hour of the turn. North-westerly to north-easterly winds blow straight into the entrance and, against an ebb tide, create steep breaking seas over the bar and Hebe Reef. Southerly winds blow offshore and give the calmest water.

Visibility is best on the flood tide, when clear Bass Strait water enters; the ebb brings turbid river water, especially after rain. Slack water only.

Descend on a shot at slack, circle the rock at 25-30 m looking at the sponges, and ascend promptly on the shot. Listen for approaching ships. Turn the dive at half gas (thirds where current or overhead sections are involved), deploy an SMB before surfacing for the live-boat pick-up, and complete a 3-minute safety stop at about 5 m where depth allows.

Tasmanian Divers (42 Invermay Road, Launceston) runs regular local dives at northern shore sites including the Tamar region; Launceston clubs (Tas Uni Dive Club Northern Chapter and others) dive Low Head and the Tamar heads. No charter boat is based at Low Head; boat sites are dived from private boats launched at Low Head, George Town or Kelso.