3-Day Cruises from San Francisco
One cruise line offers voyages from San Francisco that are just 3 days long. The departures out of SF are in the spring when a ship is repositioning north. Quick 3-day cruises are popular because they are easy to fit into a schedule and they are affordable.
The video below is a good introduction to a 3-day cruise. It's only 3.5 minutes long.
A 3-day cruise is a good way to try out cruising if you've never been before. You'll get to experience all of the entertainment and food without committing too much time or money.
Only one 3-day cruise from SF is available. It is scheduled for three days from Saturday to Monday (2 nights). It is the only short voyage option.
The short one-way cruise from San Francisco to Vancouver is on Holland America Line. It departs May 5, 2018. That is a Saturday and the ship leaves at 5:00 PM. Sunday is a day at sea to enjoy all of the amenities the ship has to offer. You'll arrive at Vancouver at 11:00 AM on Monday. This cruise makes no stops in route.
The thing that prevents 3-day cruises from San Francisco is the federal law that mandates that ships must stop in a foreign port. There aren't any foreign ports close enough to complete that quick round-trip.
For other options, see 3-day cruises out of LA. That is the main departure port in California for really short cruises, and they have departures all year.