Officers from the Los Angeles Police Department were dispatched to find the unnamed man after he ran away from a mental health hospital about 2:30 p.m. Saturday.
The man, who was naked and had a catheter and I.V. tube inserted, ran from officers and jumped into the harbor just west of the 4400 block of Admiralty Way near Tony P's Dockside Grill restaurant.
Sheriff's deputies from the Marina del Rey station arrived by car and boat to assist officers.
Authorities tried to coax the man onto nearby docks and into patrol vessels unsuccessfully. The man eventually became cold and tired from the prolonged exposure to chilly water and climbed onto the swim step of one of the sheriff's patrol boats.
The man was evaluated by paramedics then turned over to LAPD officers.