Lucas Horan, 39, was once remaining noticed on December 24 swimming from a ship alongside the coastline close to the Emeryville Marina.
Horan has now not been heard from since, police say.
He was once reportedly dressed in a grey fleece jacket, unknown colour blouse and denims.
Any person with details about his whereabouts is requested to name the Emeryville Police Division at 510-596-3700.
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