I'm not sure a bout storage on the docks especially with cruises coming and going but I'm sure like most ports there is a place...... I've never left from Seattle but will be doing so in late August. (You will have all the answers to my questions then right)
Pikes market is in the general area of the piers and it's a short walk across the roads,rr tracks, and up the side of the hill if pier 30 is in the middle area of the docks. Sorry I've never paid that much attention to the #'s when I've been there. Maybe a check of The Port of Seattle's website would help. ?
We will be going with locals and are just being dropped off so haven't bothered doing much research.
Good luck
I had some time and went and looked at Seattle's Port site <http://www.portseattle.org/seaport/cruise/> for cruising. If you are leaving from pier 30 there is no way you would want to walk to Pike's Market but I believe there is a shuttle/tram that goes along the waterfront (back across the street) that you could use. The area of 30 is pretty dreary as I recall.