The route for ships out of Seattle is actually NOT the Inside Passage.
The Inside Passage is the body of water that is between Vancouver Island and the mainland, as well as Queen Charlotte Islands (and other island chains) and the mainland. The most scenic part of this route is Johnston Strait, where the land is precipitous and close to the ship on both sides, and where pods of killer whales are most frequently seen. Ships out of Vancouver go up this way because it is the handiest and because shipping regulations allow them to remain in Canadian waters for longer since they embarked at a Canadian port.
The route that ships out of Seattle take is around the west side of Vancouver Island. Many people report that this is not as scenic; land very far from ship, etc. Also, people have reported that there is more ocean roll because it's open water. Because Seatlle sailings must add a few hours' time onto the route, going straight up without going through the Inside Passage means they can put the pedal to the metal, as it were.
When my husband and I did Inside Passage last year on Radiance, Johnston Strait was definitely the highlight of the trip, both coming and going. The wildlife through there was non-stop. If cruise lines are marketing cruises out of Seattle as "Inside Passage," don't believe them!