Discovery Bay Marine National Park protects 2,770 hectares of the wildest coast in Victoria, running along the shore west of Cape Bridgewater toward the great sweep of Discovery Bay beach and Nelson. It is a place of extremes. Inshore, basalt reefs and rock platforms fringe the ends of the park and the beach between is a continuous surf zone. Offshore, in 30–60 m, lie low-profile reefs formed from ancient shorelines and capped with volcanic basalt, and on them, in Parks Victoria's description, rich sponge gardens composed of thick growths of sessile invertebrates – sponges, ascidians, bryozoans and gorgonians – interspersed with plains of sand. The Bonney Upwelling, which brings cold, nutrient-rich water to the surface along this coast every summer, drives one of the richest food webs in southern Australia: krill swarms, schools of fish, Australian fur seals, little penguins, common dolphins, southern right and humpback whales in winter and blue whales feeding offshore from October to March, the only reliable blue whale feeding ground in Australian waters.

Parks Victoria says diving should only be undertaken by trained and experienced divers, that swimmers and snorkellers must beware of strong currents and undertows, and that some areas are impassable at high tide; the offshore sponge reefs are known from research surveys rather than recreational dives, and no operator visits. Whites Beach, the most accessible part of the park, offers a shore entry beside inshore basalt reef in exceptional calm, and the Blowholes and Petrified Forest on the western side of Cape Bridgewater – the limestone tubes of the Petrified Forest are in fact solution pipes, not trees – give clifftop views of the park's eastern end but no way into the water. The Glenelg River mouth at Nelson, 65 km west at the South Australian border, is a shallow bar with a river ramp and no documented sea diving. For a well-equipped expedition with a good boat and a long autumn calm, the deep reefs off Discovery Bay are one of the last unexplored dive frontiers in Victoria; for everyone else the park is a place to watch whales and seals from the cliffs.

From Portland take the Bridgewater Road west to Cape Bridgewater (21 km), then The Blowholes Road and Scenic Drive around the western side of the cape; Whites Beach, which Parks Victoria calls the most accessible part of the park, is reached from the Portland–Nelson Road and Whites Beach Road with a walk down to the sand. The Blowholes and Petrified Forest car parks give clifftop access to the park's eastern end but no practical way into the water. Boats launch at the Cape Bridgewater beach ramp or Portland Harbour and round Cape Bridgewater into the open Southern Ocean – a serious passage.

Nelson, on the Glenelg River 65 km west, has a river ramp but the river mouth is a shallow bar.

This is the most exposed coast in Victoria, facing south-west into the Southern Ocean with nothing between it and the Antarctic, and Discovery Bay's 50 km beach is a continuous surf zone with strong currents and undertows. Shore entry at Whites Beach is possible only in a rare flat calm with swell under about 0.5 m and offshore north-easterly winds, and even then rips run; Parks Victoria warns of strong currents, undertows and large unexpected waves. The offshore reefs in 30–60 m need a multi-day high with swell under 1 m and a capable boat.

The Bonney Upwelling drives cold (13–15 °C), nutrient-rich water onto this coast from November to April, feeding blue whales but reducing visibility; algal blooms occur naturally and discoloured water should be avoided. A few days a year.

Shore: at Whites Beach in flat calm, enter beside the basalt reef at the beach end, explore the inshore ledges and kelp in 2–8 m within a short swim of the exit, and return before any change in the sea. Boat: on the deep offshore reefs, anchor on sand beside a low reef ridge in 30–40 m, keep bottom time short on EAN, work along the sponge-covered ridge and ascend the line with a full stop schedule; plan for current and a drifting deco station.

Offshore reefs at 30–60 m are technical dives (EAN/trimix, staged deco, drifting station, surface support); no recreational infrastructure exists.

No operator. Parks Victoria states diving should only be undertaken by trained and experienced divers; the offshore reefs have been surveyed by researchers rather than dived recreationally. Portland SCUBA is the only nearby dive business.