Where in Northern Cali? Im from the east coast, but am a wizard finding stuff online.
Head over to Searchtempest.com, type in your zip and put a 400 mile radius. Search for tons of keywords like 750sx, kawasaki jet ski, stand up jet ski....blah blah blah. If the person that stole it tries to sell it, it should be too hard to track down. If you are not familiar with searchtempest, it searches all craigslist ads within a given range.
Sorry to hear, but good luck!