Why do many cities near the california coast begin with san or santa?

That phenomenon is not really connected with being near the coast. 
It's more a matter of places that were once conquered and re-named,
mostly by the Spaniards.
 
Conquest and subjugation of the local indigenous population, you see,
was a brave, heroic, macho accomplishment, which made them really
feel good.  So when they erased the names that the local people had
given the places, they very often re-named the places in honor of their
king, or in honor of religious figures such as Saints ... "San" for a male
saint, and "Santa" for a female one.

At great distances from the coasts, you have San Antonio, Santa Fe,
Saint Louis, etc.