I think the farthest north was once or twice to Lake San Antonio, a bit north of Paso Robles. But a caravan from So Cal to either the Sacto delta or Shasta would be pretty freakin' cool to do sometime.
I don't lock my boat but it takes quite a bit of practice to back up to it just right, to where you can hook up. But chance of theft of a 40 year old boat don't worry me much. That's why I pay insurance.