Lagoon Pier is the northernmost of the Port Phillip pier dives and one of the closest pieces of diveable structure to the Melbourne CBD. Built in 1889 at the head of Hobsons Bay, it runs roughly 300 m out from the Port Melbourne beach and is unusual among the bay's piers for being narrow and slightly curved, which gives the dive a different feel from the broad, square timber decks at Brighton or Mordialloc. The name records a lagoon that once ran inland from here and was progressively filled by 1929; the adjacent land was the HMAS Lonsdale naval depot until the 1990s and is now apartments.

The pier is a public berthing pier, so small craft tie up along it for up to 48 hours. Underwater it is a shallow silt-and-sand dive. Depth is barely a metre at the beach and only reaches about 4 m at the outer end, so the whole dive is spent among the pylons.

The piles carry a modest growth of mussels, tubeworms, sea squirts and bryozoans, with the usual Port Phillip pier tenants: Tasmanian blennies in the crevices, southern cardinalfish, small leatherjackets, toadfish, decorator crabs and the occasional seahorse or pipefish on the weed beds beside the structure. Look under the odd piece of debris on the sand for octopus. Because the bottom is fine mud, visibility is easily wrecked by careless finning and is rarely more than a few metres even on a good day.

It is not a destination dive. Its value is as a quick, safe, very shallow site for checking new gear, running a first night dive, practising navigation, or snorkelling with children, and for divers living in the inner suburbs who want to get wet in half an hour rather than drive to the peninsula. Combine it with Kerferd Road Pier, a kilometre to the east, for a two-tank morning.

From Melbourne CBD take Clarendon Street or Bay Street south to Beach Street, Port Melbourne (about 15 minutes). Park in the car park off Beach Street opposite Esplanade East; it fills quickly on warm weekends and is metered. The pier is a short flat walk of under 100 m from the car park, and you can gear up at the car and walk down onto the sand.

Enter from the beach on either side of the pier (the south side is usually used) and swim out along the pylons. The water is very shallow for the first 50 m, so fin on the surface until it is deep enough to descend. Exit back up the beach.

The yacht club and life saving club both have kiosks.

Lagoon Pier faces south-west into Hobsons Bay and picks up moderate wind chop in any strong southerly or south-westerly; it is blown out by anything over about 15 knots from the south quarter. It is at its best in northerly, north-easterly, north-westerly or westerly winds, or light south, south-west or east winds. Visibility is generally poor (2-4 m) because of the Yarra outflow and fine silt, and drops further for several days after heavy rain.

Dive on the top of the tide for the best depth and clarity; winter often gives the clearest water. There is no swell to speak of. Night dives are popular as the pier is lit and close to town.

Enter from the beach on the south side, fin out on the surface to the third or fourth set of pylons, descend and work slowly out along one row of piles to the end (about 4 m), cross under the pier and return along the other side. Keep a metre off the bottom to avoid silting out. Turn at half your gas or 30 minutes and ascend in the shallows beside the beach; no safety stop is needed at these depths.

No regular commercial operator – dived independently and by Melbourne dive clubs and shop social-dive programs (Scuba Culture, Aquability, Melbourne Diving Services run occasional shore-dive nights at bayside piers); Ricketts Point-based snorkel tours run separately.