THIS ONLY DISCUSSES HOW TO GET OUT OF SEATTLE - WE DIDN"T SPEND MUCH TIME IN THE TOWN ITSELF.
You are in for a wonderful ride. I envy you. Make sure to take a camera, because you are going to get some spectacular shots.
I'm not sure how you are getting into Seattle. We came into Seattle on AmTrak. We were NOT impressed with the train station. It was being remuddled, and the staff were unfriendly. If we had been forced to assemble our bikes in Seattle rather than Portland the night before, I'm not sure how it would have gone.
I have no experience with SEATAC. But riding to the ferries seems pretty straight forward.
The ACA maps for the Pacific Coast take you from Brimmerton, Washington, to North Bend, Oregon. We took the ferry from Seattle to Brimmerton (about an hour), and then rode to the west side of town, where we spent the night in a pretty good ACA suggested motel. We were able to duck under the highway, and catch the ACA route. It takes you down, roughtly parallel to I-5 down to Longview, Washington, where it takes a right at the Columbia River, and on the Cathlamet ferry. Once across the river on the ferry, US-30 takes you in to Astoria. And, from there, it is down US-101 to North Bend.
We loved the Oregon coast, especially the Otter Crest Loop. The Washington State route had very little coast line south of Seattle. And, to our good fortune, last June was the wettest June in some time, with 200% of normal.
If I we have a do over, we will strike out for the coast from Portland, rather than starting in Seattle. As it turns out, because my wife's rear hub starting making "funny noises", we took the train from Centralia Washington to Vancouver Washington, where the guys did their magic and charged us a pittance. TO get back on track, we rode side roads from Vancouver to Kelso (the only strenuous part was Green Mountain Road which was a bear). From there, we were able to rejoin the ACA route.