The Marietta Dal was a freighter that UniDive describes as a Liberty ship built at the end of the Second World War. In 1950 she ran aground on Smith Rock off Cape Moreton carrying a mixed cargo of oils, chemicals, machinery and 76 tractors. She broke in two, and attempts at salvage were abandoned because of the heavy seas that regularly batter the rock.

Today she lies in 12-15 m, with the hull resting flat on the ocean floor at about 13 m and the cargo scattered around it. This is the shallowest and most accessible of the three Smith Rock wrecks and suits open-water divers when the sea is calm. The highlights are the cargo and the machinery: the farm tractors are still recognisable among the wreckage, and the giant propeller shaft, about 50 m long, runs across the site and makes an easy navigation line.

Plates, frames and machinery form a maze of shelter for fish, and the wreck is well grown with sponges and corals. Schools of reef fish, rays, wobbegongs and crayfish are typical, with pelagic fish passing through from the surrounding open water. The shallow depth gives plenty of bottom time, but it also means the site feels every bit of swell passing over it, so surge is the main limitation.

Many divers combine the Marietta Dal with a dive on Smith Rock itself or with Flinders Reef. It is a good introduction to wreck diving outside the bay and an interesting piece of post-war maritime history.

Boat access only. Brisbane charters leave from Manly Boat Harbour (Brisbane Scuba departs from the corner of Royal Esplanade and Davenport Drive, Manly) or from Scarborough Boat Harbour on the Redcliffe Peninsula, crossing Moreton Bay and running out through the North West Channel past Bulwer and Comboyuro Point to the northern tip of Moreton Island. Allow roughly 75-90 minutes from Manly in good conditions.

Sunshine Coast operators run south from The Wharf, Mooloolaba, in about 45 minutes. Private boats typically launch at Scarborough or Bribie Island; the open-water crossing around Cape Moreton is exposed and should only be attempted in settled weather with local knowledge. Smith Rock lies between Cape Moreton and Flinders Reef, a short run from the cape; the three historic wrecks lie on and around it.

The wreck is shallow and exposed to swell rolling past Cape Moreton; UniDive warns surge can be challenging. Swell over about 1.2-1.5 m from the east or south-east makes the site uncomfortable. The area is tidal, with currents running past Smith Rock, so slack water is preferred.

Visibility averages about 20 m and is best in winter.

Descend onto the flattened hull at about 13 m, find the long propeller shaft and follow it, then explore the scattered cargo of tractors and machinery. Stay low in the wreckage in surge and return to the line.

Brisbane operators (Brisbane Scuba, Go Dive Brisbane) and Supercat Charters – occasional outer-Moreton days; UniDive club trips.