One dead, two rescued after boat overturns offshore

One person is dead and two have been rescued after their boat capsized about 20 kilometres off the West Australian coast.

Emergency services responded to reports of an upturned vessel with three occupants in the water shortly before midday on Friday.

Police believe the friends were fishing in the roughly five metre boat which was anchored when they experienced difficulty.

It was located by another fishing vessel roughly 10 nautical miles west of Perth suburb Alkimos, which then alerted authorities.

Senior Sergeant Jamie Brennan of Yanchep police said it was believed the man was already deceased when the second vessel arrived.

The two survivors were in good physical shape despite spending around two hours in the water, Snr Sgt Brennan said.

"They're obviously shaken as one would expect having lost one of their friends," he said.

"There was a fair bit of swell out there today. The sea can be quite an unforgiving environment, so you've always got to have your wits about you."

The two survivors were wearing life jackets and an orange smoke flare was deployed to attract help, police said.

It is not known whether the dead man had been wearing a life jacket.

RAC helicopters, along with water police and marine rescue were involved in the effort.

The vessel will be seized by police as part of an ongoing investigation and a report is to be prepared for the coroner.