A man has been found dead about 18km off the coast of Western Australia after a boat capsized.

The man was found dead in water alongside two other men, who were rescued after being found near the upturned vessel.

Western Australia Police said the boat was about 18km from Alkimos, north of Perth.

Senior Sergeant Jamie Brennan said the men were on a fishing trip when they ran into trouble.

A man has been found dead about 18km off the coast of Western Australia after a boat capsized (file picture)

'The two survivors and the deceased were returned to the shore and the capsized vessel was also towed back here where it was seized and part of the ongoing investigation,' Snr Sgt Brennan said.

'Interim information received would indicate that the vessel was anchored, they came into some difficulty, and shortly thereafter, the vehicle capsized.

'They did have emergency equipment on board and the survivors were wearing life jackets and an orange smoke flare was deployed.'

Snr Sgt Brennan said he'd spoken to the two survivors.

The man was found dead in water alongside two other men, who were rescued after being found near the upturned vessel.

'They're obviously shaken as one would expect, having lost one of their friends, however physically they appear to be okay,' he said.

'There was a fair bit of swell out there today ... the sea can be an unforgiving environment.'

A report will be prepared for the coroner.