Back in the 1980s we were still able to swim through the Cerberus, Victoria's first ironclad warship, which ended it's life as a breakwater in Half Moon Bay, Black Rock. The inner and outer hull were still relatively intact, and we were able to move along inside the spaces between the hulls and also explore the inside of the hull.
More recently, much of the hull has collapsed and compressed, making it a dangerous place to dive, and the hulk has become a restricted area.
This used to be our favourite mid-week night dive. The pier, while not long, did attract quite a bit of life, and the Cerberus was always an interesting and exiting place to explore.