Police from the Hawkesbury region and NSW Ambulance paramedics were called to the Colo River, near Putty Road, about 11.50am on Sunday after reports that a man was pulled face-down and unconscious from the water.

Witnesses tried to revive the 32-year-old man before paramedics arrived, but he was pronounced dead at the scene. A report on his death is being prepared for the coroner.

Paramedics were later called to Wakehurst Parkway in Narrabeen, about an hour and a half south-east of Colo, after reports that a young girl and boy had got into trouble in the sea.

Three ambulance crews and the CareFlight helicopter arrived after receiving a call at 12.45pm, and performed CPR on an unconscious seven-year-old girl.

She was taken to the Sydney Children's Hospital in a serious but stable condition, while a five-year-old boy, pulled conscious from the water, was taken to the hospital as a precautionary measure.

A three-year-old girl was with the other two children but did not have to be rescued.

The incidents come just days after State Emergency Services launched their annual Summer Safety Campaign.

NSW Surf Life Saving chief executive Grant Pearce urged people to take proper precautions in NSW waterways, including swimming at patrolled beaches, and doing so in between the red and yellow flags.

"Each summer our lifesavers respond to many coastal drownings. Most are away from a patrolled location or after patrol hours, so make sure you learn how to spot and avoid rip currents, and don’t overestimate your abilities in the surf," Mr Pearce said.

The Royal Life Saving National Drowning Report showed that 291 people drowned in Australian waterways last year, with 23 per cent of those drownings occurring in rivers, creeks and streams.

The other main locations where drownings occurred were beaches, at 17 per cent, and oceans or harbours, at 16 per cent.