Possibly a silly question - but can you sail / cruise from San Francisco to Seattle - or even to vancouver?

I assume you mean by a cruiseline or similar?

California to Vancouver might be possible if there's a repositioning cruise (spring) - I'm not familiar with any but haven't looked. Check out a big cruise website like cruise.com; you might find something.

California to Seattle would only be possible with a US-flagged vessel, of which there are very very few in the cruise business. Doing this trip on a foreign-registered ship runs afoul of the "cabotage" laws preventing intra-US transportation on foreign ships unless an intermediate "distant foreign port" is visited. By the way, neither Canada nor Mexico qualify as "distant" foreign ports. One way trips from non-US to US ports (SF to Vancouver, for example) is kosher for foreign boats. Or, if the trip is a round trip (SF to SF, say) then an intermediate stop in any foreign port (like Victoria BC) is also legal on foreign ships. Confused yet?

This is why the one-way Alaska cruises all start or end in Vancouver, or the Hawaii one-ways go to Ensenada, or the round trips to Hawaii from LA or Alaska from Seattle all stop for a few hours in Mexico or Canada respectively. Major pain.

I doubt it's possible to Seattle as I've heard something about the law of visiting the ports of different countries.

I remember seeing 2-day "love boats" or some similar name of those short tours, or there may be longer ones, like round-trips.

To start I suggest go to a website like Expedia, search for cruises out of SF.

Also check with the cruise forum, they know everything, even how to smuggle wine and soda on board, I kid you not!