Portsea Pier is the western-most of the bay piers and the one most influenced by the ocean: sitting 3 km inside Port Phillip Heads, it is flushed by clean Bass Strait water on every tide and consequently has better visibility, denser kelp and a more southern-ocean feel than the piers to the east. The L-shaped structure runs about 300 m out over sand and patches of seagrass and kelp, with 3 m at the head, and because the deck is only 4 m wide the bottom is well lit all along its length. It is the most reliable place on the bay side to see weedy seadragons, which drift in the kelp and seagrass around the head and the outer pylons, and it is equally good for blue-ringed octopus, sea hares, nudibranchs, fiddler rays, biscuit seastars, puffers and bluespotted goatfish.

Old tyres on the sand shelter octopus and crabs, and soft corals and sponges coat the pylons. Fifty metres either side of the pier lie small reefs – Portsea Pier East Reef, which stretches under the moorings toward Point Franklin to 11 m, and a West Reef – where juvenile fish and small critters gather and which experienced divers add to the pier dive when the tide allows; Portsea Cray Reef lies a further 180 m north-east in 5–12 m. Dives here are unhurried and typically last well over an hour.

The pier's character is shaped by the tide and the traffic: the tide brings surge and a gentle current on the run, the Sorrento ferry and summer boats pass close, and at night the deck is lined with squid fishers whose jigs hang in the water column. Parking is tight by day in summer, so many locals dive it in the evening. Portsea suits every level, is a fine training site, an outstanding macro and seadragon site, and a deservedly famous night dive; it is also the shore diver's gateway to the Point Nepean reefs to the west.

Portsea Pier is on Point Nepean Road in Portsea village, 100 km from Melbourne, 4 km past Sorrento. Parking is the constraint: spaces within 50 m of the pier are limited to 1–2 hours during the day in summer, are unrestricted from May to November and are free after 6 pm year-round, so evening dives are easiest. The best entry is from the shore on the western (left-hand) side, where a path leads down onto the beach; swim out along the pylons.

Exit up the ladder on the lower landing or back onto the beach. The pier is only about 4 m wide, so light reaches the bottom throughout. Bayplay's dive shop and hire are a few minutes' walk away.

Portsea Pier is best on light offshore southerly winds and a high tide; it faces north-north-west and strong northerlies or north-westerlies put chop and surge along the pylons. Being only 3 km inside the Heads it feels the tide more than the piers to the east – since the 2009 channel-deepening the source notes increased swell and surge on running tides – so slack water is preferable though not essential, and divers should expect a gentle push along the pier when the tide is moving. Visibility is typically better than at Rye or Blairgowrie because clean Bass Strait water floods in on each tide.

The Sorrento–Queenscliff ferry and heavy summer boat traffic pass close; squid fishers crowd the pier at night.

Enter from the western beach, descend by the first pylons in 2 m and follow the western pylon row out to the L at the head in 5–7 m, where the kelp and seagrass beds begin. Explore the head and, if slack, the West Reef 50 m out, then return under the pier along the eastern row, checking the tyres and pylon bases for seadragons and blue-ringed octopus. Finish in the shallows and exit via the lower-landing ladder.

Bayplay (Portsea) runs guided dives, night dives and courses here daily in season; Dive Victoria (Portsea) and The Scuba Doctor (Rye) dive it regularly; one of the five most-dived sites in Melbourne. No charter needed.