Emergency services were called to Long Reef off Anzac Parade, Collaroy, after 9am on Sunday following reports a boat had overturned.
Three people were thrown into the water.
Police from Northern Beaches Police Area Command attended, assisted by the Marine Area Command, Surf Life Saving NSW and NSW Ambulance paramedics.
The body of a man, believed to be in his 60s, was found in the water. He is yet to be formally identified.
A man, 33, and boy, 23, were rescued from the rocks nearby and were being assessed by paramedics.
It is understood the older man and child were wearing life jackets.