Bongin Bongin Bay, which everyone in Mona Vale calls the Basin, is the small crescent of sand tucked under the northern side of Mona Vale headland between the golf course and the rock platform, a favourite swimming and snorkelling spot because the headland shelters it from the southerly swell that pounds the main beach. Vizzbud describes 'mixed sand and reef' with the reef 'dropping to around 12–15 metres at the sand edge', an 8 m average, an easy sandy entry and sheltered conditions that make it suitable for newer divers, and lists seahorses, sea dragons and cuttlefish as the commonly sighted animals. The bay's floor is sand and seagrass close in, with the reef running out along the northern headland as boulders and kelp to the sand line; the sheltered water lets seagrass persist, which is why seahorses are found here where they are absent from the exposed beaches.

Weedy sea dragons live along the kelp margin at depth, cuttlefish and octopus in the rocks, stingarees and flathead on the sand, and the headland reef carries the usual blue groper, wobbegongs and Port Jackson sharks in winter. Visibility is best on winter westerlies. It is the most beginner-friendly ocean-side dive north of Long Reef, an easy snorkel, and a good night dive, and it has a dive shop a few minutes away.

The cautions are minor: surge on the headland platform in a northerly, stormwater after rain, and boats anchoring in the bay on summer weekends. It suits Open Water divers, families and photographers, and combines with the Mona Vale headland platform dive on the southern side. Details are from Vizzbud and should be confirmed on site.

From Pittwater Road, Mona Vale, park free on Golf Avenue and the adjacent streets beside Mona Vale Golf Course and walk a short distance down to the Basin beach (Bongin Bongin Bay) between the golf course and the headland. Enter from the sand at the northern end of the bay and swim out over the mixed sand and reef, or use the rock platform on the northern headland in flat conditions. Exit onto the beach.

The beaches between Narrabeen and Palm Beach face east to north-east and are open to the full Tasman swell; anything over about 1 m from the north-east or east makes the rock-platform entries a wash zone and puts surge across the shallow reefs, and a southerly swell wraps around the headlands. Choose mornings with under 1 m swell and a westerly or light north-easterly, on the top half of the tide so the platform edges are covered. Visibility is 5–12 m on winter westerlies and drops after rain when the lagoons open.

There is no dive shop north of Brookvale except Dive Spear and Sport at Mona Vale, so fills and advice are limited; these are independent divers' sites. The Basin faces north-east and is sheltered by the Mona Vale headland from southerly swell, making it the calm-weather option on this stretch; a northerly swell puts surge on the headland platform and stormwater run-off after rain reduces visibility. Boat traffic uses the bay in summer.

Enter from the northern end of the beach, descend over sand and seagrass at 3–5 m searching for seahorses, and follow the reef out along the northern headland to the sand edge at 12–15 m where sea dragons and cuttlefish are found; return along the inner reef at 6–8 m and finish over the sand.

No scheduled operator; Dive Spear and Sport (Mona Vale) for fills and local dives; independent divers and snorkellers.