The Denison Canal at Dunalley was cut through the narrow isthmus between Blackman Bay and Norfolk Bay in the early twentieth century to save coastal vessels the long voyage around the Tasman Peninsula, and it is still maintained for boat traffic today, with a swing bridge carrying the Arthur Highway. The Marine Life Network guide calls it the only easy drift dive in south-east Tasmania: the tidal stream through the canal can be fast, carrying divers along the muddy bottom at about 10 m. Schools of soldier fish (a local name, probably for bullseyes or cardinalfish) shelter along the canal walls, and in season spider crabs gather in mating congregations – an extraordinary spectacle when hundreds or thousands of crabs pile on top of each other.

The bottom is mud with scattered debris, and there are the usual estuarine species such as flathead, sea stars and small crabs. The dive is strictly controlled: the guide says divers need Harbour Master permission and a manned safety boat that follows and picks them up, because the canal is an active navigation channel. Visibility is usually modest.

For a group with the right arrangements, the canal provides a fun and unusual dive. Without them, it should not be attempted. Dunalley has a pub, cafe and bakery, and makes a convenient stop on the way to the Tasman Peninsula.

Divers interested in the site should contact TasPorts or Marine and Safety Tasmania well in advance, and dive with a club that has done it before. Spider crab aggregations elsewhere in southern Australia typically occur in late autumn and winter, so ask locally about timing.

Dunalley is on the Arthur Highway about 55 km (45 minutes) east of Hobart via Sorell. The canal runs beside the town under the swing bridge. The Marine Life Network guide stresses that diving requires permission from the Harbour Master (now TasPorts/MAST) and a manned safety boat that follows divers and picks them up at the end, or divers swim to shore.

Launch at the Dunalley boat ramp. Never dive the canal while vessels are transiting or the bridge is opening.

The canal is sheltered from swell, so weather matters less than tide. The tidal difference between Blackman Bay and Norfolk Bay drives a fast current through the canal – the Marine Life Network describes it as 'quite fast'. Plan the dive for the direction and strength of the stream you want, with the safety boat following.

Strong winds make boat handling in the narrow channel harder. Visibility is typically low over the muddy bottom.

Enter at the up-current end with the safety boat in attendance, descend to the bottom at 8-10 m, and drift the length of the canal together as a group with a surface marker. Surface at the far end for boat pick-up or swim to the bank.

Requires prior approval from the port authority; dive only when no vessel movements are scheduled.

No regular operator; occasional club dives with Harbour Master clearance and a safety boat.