I'm certainly no expert on Seattle's stormwater system but here's what I know based on living in the area. All of this is with the very large caveat that things are probably different 60 years in the future and that the GM could have things work however he/she wants.
Seattle does not treat its stormwater. Most of it goes directly into Elliott Bay / Puget Sound. Seattle treats its wastewater at the West Point Treatment Plan in Discovery Park. I think the ACHE and the Aztechnology Pyramid have their own treatment plants. I don't know how many other buildings would as well. The sewers under Current day downtown Seattle streets are definitely large enough to be navigable by people.
Everett absolutely has docks. Conveniently, Everett's wastewater treatment center is 3km directly east of those docks. This plant treats both the stormwater and the wastewater collected from the downtown Everett area. It's called the "Combined Sewer area". See
this page. Check out the map; the light blue portion on the north end is downtown Everett. The west side of that portion is the docks; the east side of that (past I-5), directly east of the docks, is Everett Water Pollution Control Facility. Based on the amount of rain in Everett, and the fact that their combined sewer system must account for wastewater too, I would definitely expect the sewers in downtown Everett to be large enough for metahuman traffic. Just don't go when it rains.
Conclusion: The combined sewer system of Everett should be large enough for a team of shadowrunners to bypass the perimeter of the docks. But gross.
P.S. Most of what Umidori says is true, although I only know of Seattle burning to the ground once. There were also some lols with the ladies having to ascend and descend ladders to and from street level, and gentlemen that would guard the bottom of the ladder from less-gallant types looking for an upskirt view.