The victim, 41, "felt bad in the water, became unconscious, was removed from the water to the beach by surfers, was subjected to resuscitation maneuvers, but without success," Paulo Agostinho told Lusa news agency.
According to the commander of the captaincy of Cascais, the man had health problems and there had already been a similar episode 15 days ago.
So, he added, only the autopsy will be able to determine whether the death was caused by drowning.
The alert was given at 1:30pm, and 10 operatives and five vehicles of the Maritime Police, elements and resources of the National Institute of Medical Emergency and firefighters were mobilised to the scene, including two ambulances and a jet ski from the Ericeira lifeguard station.
The head of the Cascais captaincy also said that the good weather and the easing of lockdown measures are attracting citizens to the beaches, but warned that most have no bathing surveillance.