It's 5MPH within 200' of any swimming beach, dock, swimming platform or landing where boats are tied up. It's 100' from any person in the water excluding water skier.
Here is a link to the ABC's of California Boating.
http://www.dbw.ca.gov/Pubs/Abc/
IMO this is one of the reasons the river sucks. Most of it is not 200' wide and you have private docks every 50'. I say fock-em. If they can afford a house on the river then they should get a lift for their boat.