I went up "all the way" several decades ago in a boat twice the size of my C Dory. I remember coming to a low bridge. The tide had changed and there were ripples on the water from the river current. With the tide dropping, I called it a day. I could probably get under the bridge and continue on with my 16, but . . . The most interesting stuff that I saw was in the first 1.5 miles. And it was interesting enough that doing the Duwamish was fun. That is assuming that you like looking at gigantic commercial vessels, gritty industrial areas, monstrous loading facilities, etc. It's a scenic river, but not in the normal way.
Mark