It's just like any other cruise - full buffet, show ...

It's a way for the cruise line to provide a tour of their ship, so that one can decide to stay on those same ship for a longer cruise.

Besides, the Seattle-Vancouver cruises are technically repositionning cruises, which are generally cheaper as well.

You can find them here:
US to Canada: http://www.cruisetosomewhere.com/from-us-to-canada.htm
Canada to US: http://www.cruisetosomewhere.com/from-canada.htm