Try the South Park Marina. It's on the Duwamish river, off Elliot Bay, just south of downtown Seattle, (access is easy, and there are no locks to traverse). They have some of the lowest rates in the area and can provide just about any service you might need. Slips go for $7.00 per foot (I pay $11.00 on lake Union) Lots of sailboats on the hard, with lots of locals to help with questions etc. As with all local boatyards, they do the initial pressure washing for you.
South Park Marina