Marengo Reefs Marine Sanctuary protects two small sandstone reefs, Little Henty and Henty, that break the surface just off Marengo beach on the southern edge of Apollo Bay. At 12.5 hectares it is one of Victoria's smallest marine protected areas, but it packs in a lot: an intertidal platform with rock pools, gutters and ledges, subtidal reef covered in bull kelp and colourful red and green algae, sponges and sea squirts on the shaded faces, and, on the outer reef, a haul-out of Australian fur seals that has made Marengo the Great Ocean Road's best-known snorkel. The seals are the draw. Young animals in particular will leave the rocks to circle, buzz and play with snorkellers and divers in the channel, and on a calm day an hour in the water with them is as good as anything at Montague Island or Phillip Island, without the boat trip. Beneath the surface the reef is typical productive Otway coast: dense kelp with abalone and southern rock lobster under the ledges, sweep, wrasse, magpie perch, zebrafish and leatherjackets in the gutters, weedy seadragons in the weed on the sand edge, and rays and flathead on the sand between the reefs. Dolphins pass through regularly.

Depths are modest, 2–5 m over most of the inner reef and 8–10 m on the outside of Henty Reef, so this is a long, shallow dive rather than a deep one, and it suits snorkellers, new divers and families as much as experienced divers. The sanctuary has been no-take since 2002 and the crayfish and abalone are correspondingly large and obvious. The catches are the exposure – both reefs are surf-swept in any real swell and the channel runs a current – and the seals themselves, which are wild animals that can be aggressive on the rocks; Parks Victoria asks visitors not to walk over exposed reef toward them and to stay in the water rather than land. Boat tours from the Apollo Bay harbour and guided kayak-snorkel trips run through the summer for those who prefer not to swim out from the beach.

From Apollo Bay drive 3 km south on the Great Ocean Road to Marengo and turn left into Marengo Crescent or the Marengo Holiday Park entrance; there is a small car park and beach access at Hayley Point at the southern end of the beach. Gear up at the car and walk 100–200 m along the sand to the point opposite the reefs. Little Henty Reef is less than 50 m off the platform and Henty Reef a further 200–300 m out; swim across the channel on the surface.

Boats and kayaks launch at the Apollo Bay harbour ramp on Breakwater Road and run 10 minutes south; Apollo Bay Surf & Kayak runs guided seal kayak/snorkel tours.

Marengo beach faces south-east and the reefs take the brunt of any south-west or southerly swell, which breaks over both reefs and runs a strong current through the channel between them and the shore. Aim for swell under 1 m and light north to north-west winds; southerlies and easterlies make the crossing to the reef dangerous. Parks Victoria warns of strong currents and undertows for snorkellers.

Mid to high tide covers the platform and makes the swim easier; on very low tides the inner reef can be reached on foot but the seals must not be approached over exposed rock. Visibility is best in autumn after settled weather and poor after rain from the Barham River. Diveable perhaps half the days in summer.

Enter at Hayley Point at mid tide, swim across the channel and descend on the landward side of Little Henty Reef in 3–4 m. Circle the reef clockwise through the kelp gutters and ledges, keeping the seal haul-out on your left and letting the seals come to you rather than approaching them. If conditions allow, cross to Henty Reef and explore its deeper outer edge to 8–10 m, then return across the channel with plenty of gas and surface close to the point to avoid drifting down the beach.

Apollo Bay Surf & Kayak – guided seal kayak and snorkel tours most days in summer; Apollo Bay Fishing & Adventure Tours / local charters take snorkellers to the seals; no scuba operator in town – divers come independently or with Geelong/Warrnambool shops.