HistoryLink is a good starting place for finding information about Seattle History. It includes entries on events, people, significant buildings and more. You may also find the printed HistoryLink's Seattle & King County Timeline useful.
The Seattle Public Library's Neighborhood History Project brings together digitized primary sources from SPL's collection and links to other resources including HistoryLink's thumbnail histories. The following neighborhoods touch on the waterfront.
Information about a specific property in King County is available online. For more tips on researching property history see Seattle Public Library's guide, Researching the History of Seattle Buildings.
Also search these online encyclopedia collections: Gale Virtual Reference & Oxford Reference Online