Cunningham Pier is the centrepiece of the Geelong waterfront, a broad pier built in the 1850s for the wool trade and later carrying a railway, now topped by a function centre and restaurant. Underneath it is Geelong's main pier dive and, honestly described by The Scuba Doctor, a dive of introduced pests and stinking anaerobic mud. The depth is 3 m under the deck and 10 m off either side, which makes it the deepest of the Port Phillip pier dives, and the piles are heavily grown: silt-covered sponges, ascidians and sea cucumbers cling to the timber and struggle against the Northern Pacific seastars and sabellid fan worms that have colonised Corio Bay since the 1980s.

That struggle is part of the interest; the pier is a living demonstration of what introduced species have done to the inner bay. Native life persists among it: blennies, cardinalfish, gobies, leatherjackets, toadfish, decorator crabs, nudibranchs on the sponges, seahorses on the outer piles for those who search, octopus in the debris and rays on the mud edges. The deeper outer piles are the most productive and the least silted.

Nearby, the remains of the Steampacket and Yarra piers and the wreck of the clipper Lightning (burnt at the pier in 1869, remains 270 m east) show how much maritime history is buried in the mud here. As a dive it suits Geelong locals wanting depth and structure within walking distance of coffee, night divers (the waterfront is lit), and anyone curious about the bay's ecology, and it is sheltered from the southerlies and westerlies that wreck the open-bay sites. Visibility is the price: fine mud, poor light and the smell of disturbed sediment make it a dive for careful buoyancy and dry-weather days.

Cunningham Pier is at the foot of Moorabool Street on the Geelong waterfront, an hour from Melbourne by the Princes Freeway. Park in the metered waterfront car parks off Eastern Beach Road or Western Beach Road; the walk to the pier root is 100-200 m. The pier deck is occupied by a function centre and restaurant and is private above, so enter from the water's edge beside the root at the Steampacket Gardens side and swim out under the structure.

Depth is 3 m under the pier and drops to 10 m on either side. Exit at the same point. Bay City Scuba in East Geelong is 3 km away for fills.

Sheltered from southerly and westerly winds by the city and the shape of Corio Bay, so it is diveable when the open bay and Bellarine piers are not; northerlies and north-easterlies put chop onto the waterfront. Visibility is generally poor (2-5 m) over the anaerobic mud and is best on the high tide in winter after dry weather; it deteriorates after rain. No swell.

Commercial and pleasure craft use the waterfront, so fly a flag and stay under the pier.

Enter beside the pier root, descend to 3 m under the deck and follow one row of piles out to the head, drop over the side to 8-10 m to look at the outer piles, cross under and return along the other row, staying well above the mud. Turn at half gas and surface at the root.

No regular commercial operator – dived independently and by Geelong clubs; Bay City Scuba in East Geelong is the local shop and dive club and uses the Corio Bay and Bellarine shore sites for training and club nights.