Gem Pier is Williamstown's original pier, a 145 m timber and concrete structure on Nelson Place that has been receiving ships since the 1830s and is among the oldest marine constructions in Victoria. It is now the berth for the Williamstown ferry, tourist cruises and the museum ship HMAS Castlemaine, and it is dived rarely and mostly by locals who want structure within sight of the city. The dive is a silt dive.

The bottom of Hobsons Bay here is fine Yarra mud that lifts at the slightest touch, and the golden rule of the site is to stay a metre above it; visibility is generally poor and a careless fin kick makes it nil. The pylons are the attraction: old timber and concrete piles crusted with mussels, oysters, sea squirts, tubeworms and sponges, sheltering Tasmanian blennies, southern cardinalfish, gobies, small leatherjackets and toadfish, with decorator crabs, sea stars, nudibranchs and the occasional octopus or seahorse for those who look closely, and the introduced Northern Pacific seastar and European fan worm much in evidence. Depth runs from about a metre at the shore to 5 m at the head.

The historical setting, with the Castlemaine alongside and the wooden boats of the Williamstown waterfront overhead, is much of the point. Practically, it is a site for calm, dry weather at high tide in winter, with careful attention to ferry movements at the berth. Divers wanting cleaner water and more life go 1 km south to the Point Gellibrand rock pools or 2 km west to Williamstown Beach and the Jawbone.

Williamstown is 10 km south-west of the CBD via the West Gate Bridge (about 20 minutes) or by train to Williamstown station. Gem Pier is on Nelson Place at Commonwealth Reserve; park in the metered bays along Nelson Place or Syme Street (busy on weekends) and walk 50-100 m onto the pier. Enter from the steps or a ladder on the pier away from the ferry berth and the HMAS Castlemaine museum ship, and stay under the structure.

The ferry from Southbank and tourist boats berth here, so check what is moving before entering and exiting.

Sheltered inside Hobsons Bay from most winds, though a fresh easterly or north-easterly puts chop onto the pier. The limiting factor is not wind but silt: the bottom is fine mud and visibility is poor at the best of times (1-3 m) and near zero after rain because of the Yarra outflow. Dive at the top of the tide, in winter, after a spell of dry weather, and stay at least a metre off the bottom.

No swell. Ferry and tour-boat movements are constant, so a flag and careful timing of the entry are essential.

Enter at the steps, descend beside the first pylons, follow one row out to the end at about 5 m keeping a metre above the mud, examine the growth on the piles, cross under and return along the other row. Turn at half gas at the head and surface at the steps, listening for engines before ascending.

No regular commercial operator – dived independently and by western-suburbs dive clubs and Melbourne shop social dives; no shop is based in Williamstown, so fills come from Dive Victoria Group or Scuba Culture.