This is the egg case of a Port Jackson Shark Egg Case (Heterodontus portjacksoni) on the ocean floor.
Port Jackson Sharks are oviparous, which means that the female lays eggs. The egg case is a tough, dark brown spiral about 7 cm to 8 cm wide and 15 cm long. It is common to see them washed up on beaches.
Image taken at 'The Pinnicle' dive site within the Jervis Bay Marine Park in New South Wales, Australia.