The Center for Wooden Boats is a great place to learn about Seattle's maritime history. Free boat rides on Sundays, sailing classes for adults and children as well as boat rentals and a center full of knowledgable and enthusiastic people!
Open Tuesday-Sunday, hours varying depending on the season.
This is a top highlight in Seattle for many reasons, encompassing the historic boats and importance of Puget Sound to the city. It's possible to rent kayaks here or go out on a boat. If you don't have time for that, it's amazing to just walk around the docks and photograph some of the historic, large boats around the marina.
It's adjacent to the renovated South Lake Union Park and the Museum of History and Industry, which just moved into it's new waterfront park a few years ago.
Visiting Seattle should crave tourists to visit sights on the water and this district is great. Also great is getting on a ferry and seeing the city water-side!