Rocks that float are still on Sydney northern beaches.
They are pumice stones - from the largest pumice raft seen in the past 50 years after an underwater volcano erupted off New Zealand in July 2012 form the Havre Seamount, I first noticed them late last year. This one is a Palm Beach.
The New Zealand eruption formed a 20,000 sq km raft that eventually spread about 4 million sq km as it broke up. Amazing.