What section of California plumbing code pertains to water main lines?
Title 24- California Building Standards Code, Part 5 is the California Plumbing Code. It is based on the ANSI Uniform Plumbing Code. You can view it for free on line.

I do not think there is any requirement that it be buried. If you do bury it, it must be "... not less than 12" below average local frost depth..." and "...not less than 12" below finish grade...". See Chapter 609. 

You should always ask your local Authority Having Jurisdiction.