Because of the Passenger Services Act (Jones Act) all non US registered ships must call on at least one foreign port in their itinerary. For that reason, there are no ships that only work the west coast, however it would be a great itinerary.
Most cruise lines offer sailings up and down the west coast on repositioning cruises to/from Vancouver. Celebrity has been offering sailings from San Franciso that stop in Monterey, Catalina on their way to Cabo San Lucas and this makes a nice itinerary as well.
The four day cruises to Baja out of San Pedro (Los Angeles) also stop at Catalina and San Diego.
Hope that helps.