Don't know how the law goes in California, but here in Texas erecting any structure, even temporary, or altering the lay of the land without ownership/owner permission is illegal.
From a practical point of view, moving the plants, even from a raised bed, will be difficult and probably result in a lot of plant loss. The plants will get large and be difficult to move without killing them.
Your local tax office should be able to help you determine who owns the land. I'd recommend arranging a lease agreement, even if you just pay a dollar a year, for your protection. At least get permission in writing.

There was a feature article in the San Francisco "Chronicle" a few weeks ago about guerrilla gardening. Many issues (availability of water, acquiescence or otherwise of landowner, personal safety of gardeners, etc.) were discussed both in the article and in sidebar articles.
Simply from a logistical standpoint, you might look at Mel Bartholomew's "Square Food Garden," esp. the 3' x 3' model with a "floor" containing drainage holes. This size of SFG would be very portable; in fact, "Mel" uses the 3' x 3' size for classroom and other demos!
I do agree with marlingardener that a lease agreement would help straighten out your rights AND access to water. Safety would still be a concern, however, without good fencing and other security precautions.

Im not sure what the law in California is, but there is a product out there called garden soxx that is transportable. You can lay the entire garden out on pallets and then move when needed or when the growing season is done. The garden soxx can be placed virtually anywhere and is cost effective easy gardening. In fact there is no land prep at all using the soxx. Any questions on this product, go to www.certifiederosion-nh.com and click on the growing with soxx button at the top. Good luck.

Here is a link that might be useful: Certified Erosion Control - NH