Before the Tasman Bridge opened in 1964, the Derwent was crossed by a floating bridge – a long concrete and steel pontoon structure that opened for shipping. When it was replaced, sections were towed away and scuttled; one lies off Droughty Point near a deep trench, and others rest in the Betsey Island graveyard. The Marine Life Network guide describes this section as about 580 ft (177 m) long, sunk unevenly into the mud, with the deck at about 20 m, 20-28 m at the western end and 22 m at the eastern end.

It is covered in jewel anemones and hosts mud-dwelling fish. The guide warns that visibility is low and that entering the enclosed pontoon chambers is extremely dangerous, requiring a guideline and extensive experience; the structure has been used for police low-visibility training. For recreational divers the sensible approach is an exterior-only dive along the deck and sides, with a reel for navigation in the murk.

The anemones can be spectacular in a torch beam, splashing yellow and white across the concrete. It is a niche dive for experienced local divers who enjoy low-visibility, structure-based diving and Hobart's engineering history. Because the documentation is old, check locally about its current condition and position before planning a dive.

The site suits divers already comfortable with the Betsey Island graveyard wrecks who want a more challenging structure dive close to the eastern shore. Bring a sounder, a strong primary torch and a backup, and plan to spend the whole dive on the outside of the pontoon, where the anemone growth is best anyway.

Boat access only. Launch at Lauderdale (Doran's Road ramp), about 25 minutes from Hobart via the Tasman Bridge and South Arm Highway, and run to the deep trench off Droughty Point, about 10-15 minutes. Use a sounder to find the structure; anchor off it, never into it.

Fly a dive flag and leave a boat attendant, as the Derwent carries steady recreational and commercial traffic. The Lauderdale ramp has trailer parking; fuel and supplies are at Lauderdale and Rokeby.

The Derwent estuary is sheltered from ocean swell, but strong northerly or southerly winds blow along its length and raise a short, steep chop; the Marine Life Network notes the region can become very rough in gales, even in the river. Visibility is the limiting factor: it is usually low (1-5 m) and worst after rain, when the Derwent's tannin-stained, silty floodwater spreads through the estuary. Dive after a dry spell, on a high or flood tide that brings in clearer marine water.

There is a noticeable but generally mild tidal flow. Diving is possible year-round; winter water is cold (9-11 °C).

Descend a shot to the deck at about 20 m, run a guideline along the outside of the structure, and inspect the anemone growth without entering the pontoon chambers. Ascend on the shot.

Guideline and redundant torches recommended even for exterior dives; interior penetration requires overhead-environment training and should not be attempted recreationally.

No regular operator; used historically for police low-visibility training; occasional experienced club divers.