Renting a car in Seattle to drive back over the border can turn out to be expensive. Your better options are to:
End the cruise in Vancouver. You can then take public transportation to a downtown Vancouver timeshare or there is also good bus service that takes you across the ferry (through the Gulf Islands so very scenic) to Victoria. The latter stops right in front of the Worldmark so pretty darn convenient. At that point, you can then decide if you want to rent a car for a day trip.
If you end the cruise in Seattle, you could take the Victoria Clipper from Seattle to Victoria where it's a short walk or taxi to the various timeshares. OR take the train up to Vancouver which goes along the water and is also very pretty and then a taxi into town. You really don't need a car in Vancouver and parking/traffic can be an issue so working the non-car options makes a lot of sense.
Another option is Nanoose Bay north of Nanaimo. Take the ferry from Vancouver and then rent a car in Nanaimo (there's a thread here someplace on your options for that resort).
In August, I would look at Victoria or Nanoose Bay. Vancouver could be pretty warm. Maybe add a couple of nights in town and then over to Vancouver Island. Nanoose Bay trades through RCI and provides a good base for a day trip over to Tofino and Pacific Rim National Park.
Sue