Police divers have found the body of a man who became distressed and disappeared under the water while swimming in the Hawkesbury River.

The body of the 45-year-old was found about 11.30pm on Saturday, almost five hours after he vanished while swimming across the river northwest of Sydney.

NSW Police chief inspector Garry Sims has urged swimmers to "assess the environment and know the limits of their ability".

"It is a tragedy for this to occur at this time of year and at the start of the summer season," he said.