As everyone else has already stated, you really need to look at the various methods, consider the pricing, consider the time, and consider the trouble involved in each one.
Unless I missed something, I didn't see two other options; train or bus. Both are available from Seattle to Vancouver. Also, cruise lines provide their own transfers, which can be more expensive than doing it yourself, but are easy and hassle-free.
Flying directly into Vancouver may cost more, but can be less hassle. So it's a personal choice.
We love flying into Vancouver because they have a great way of handling cruise passengers with their US Direct service for those who purchase the cruise line's transfers and are flying in the day of the cruise. We check our baggage at Dulles Airport and don't see them again until they arrive in our cabin on the ship. Very quick & easy. Of course, if you're flying in a day or two before the cruise, then this will not apply. And if you've never been to Vancouver, I definitely suggest spending at least a day or two sightseeing. It's a beautiful city with great people.