Police have found and rescued a severely dehydrated man who went on a solo snorkelling mission kilometres off the north west coast of WA.

Officers onboard a charted search plane spotted the man in knee-deep water late on Friday, April 17 - several hours after his family had alerted authorities that he had failed to return from a planned solo boating and camping trip to Picard Island.

The 28-year-old man had set off in a 3.3-metre inflatable dinghy and had been snorkelling about five kilometres from the island when his vessel broke anchor, leaving him stranded.

He attempted to swim to another nearby island and spent several hours in the water before eventually reaching land, where he remained until he was picked up by a commercial fishing vessel, which was fortuitously nearby and was able to be directed to the scene.

Officers from Roebourne and Karratha were assisted by Volunteer Marine Rescue crews, along with commercial and private vessels.

The multi-agency search from the skies was coordinated by the Water Police Coordination Centre in North Fremantle, with support from the Pilbara Police District, WA Police Emergency Operations Unit and Police Air Wing.

The man was taken to Point Samson, where St John WA paramedics treated him for severe dehydration. He was not injured.

In a statement on Sunday police reminded skippers to always plan ahead before heading offshore, monitor weather and tide conditions, carry essential safety equipment including lifejackets, an EPIRB, flares and a first aid kit, maintain reliable communication, and inform others of their travel plans and expected return time.