An adult man who may have been piloting a small inflatable boat in dense fog and heavy surf off the coastline just north of Marina del Rey harbor was found dead Monday by a passerby.

Los Angeles County Fire Department Lifeguards responded to the 5000 block of Ocean Front Walk in Marina del Rey after receiving a 911 call about the man at 1:44 p.m., according to the agency’s Twitter account. The location is near Topsail Street, about five blocks from the mouth of the harbor.

“Upon arrival, on-scene units witnessed a six- to eight-foot inflatable vessel beached and an adult male unresponsive,” the tweets read. “Attempted life-saving efforts were performed on the beach. The patient was pronounced dead by LAFD paramedics.”

Los Angeles County Sheriff’s Department Homicide Bureau detectives are conducting a death investigation, as is routine for any possibly boating-related fatality, according to a statement authored by Sheriff’s Lt. Steve Jauch.

“A passerby on the beach found the victim, an adult male, lying unresponsive on the sand, near the water’s edge,” the statement reads. “Additionally, a rigid inflatable boat was located a short distance away from where the victim was discovered.” At least one bystander attempted CPR before paramedics arrived, according to the statement.

— Joe Piasecki