A 47-year-old man has died after the small boat he was in with two others capsized in Sydney's south.
Emergency services were called to Boat Harbour at Kurnell on Wednesday morning after bystanders pulled the trio - two men and a woman - to the shore.
The three were in the water for 20 minutes before they were rescued, Surf Life Saving NSW said in a statement.
"A group of three were out on the water on Anzac Day about 9am when the small boat they were travelling in was struck by a wave, overturning it instantly," the organisation said.
It is believed all three were wearing inflatable life jackets.
By the time emergency services arrived at the remote location, however, the bystanders had started CPR on the 47-year-old and his life jacket had been removed, authorities said.
Cronulla lifeguards were first to arrive before the Westpac Life Saver Rescue Helicopter winched down an officer with a medical pack and they attempted to defibrillate the victim.
But the man died at the scene.
"It's unclear how the deceased man got into difficulty," Surf Life Saving NSW said.
The two others in the boat - the owner, a 39-year-old man, and a woman aged 26 - were taken to Sutherland Hospital suffering minor injuries.
Inspector Christopher Hill says people always want to head out and enjoy themselves on public holidays.
"(But) the message here is, obviously, wear life vests when out on watercraft ... and make sure you're aware of conditions," he told reporters.
Video footage aired by the Seven Network showed the small upturned boat washed onto a rock ledge. The ocean wasn't particularly rough at the time.
"The forecast was for reasonably good conditions so why the boat has overturned I don't know," Westpac rescue helicopter chief executive Stephen Leahy told AAP.
The capsized vessel was recovered and will be forensically examined, NSW Police said in a statement.
A crime scene was established and a report will be prepared for the coroner.
