Driving by car, it's about an hour and a half from Seattle to the border, and probably about half an hour from the border to Vancouver proper. Then, from there you'd have to take a ferry to the island; figure on a couple of hours for the crossing.

It looks like Chemainus is about equidistant from Nanaimo and Victoria (the two ferry terminals on the island), and if I were you, I'd probably opt for Victoria. It's a bit of a longer crossing, but Victoria would make for a decent base if you have car. If you're planning on being car-free the entire time, I'd pick whchever terminal is most convenient for your schedule, as you'd likely be using closer accomodations.

I believe that you can take a ferry from Washington State right up to Victoria, too, but have never done it.