The Center for Wooden Boats in Seattle has always offered free Sunday sails around Lake Union. During the month of August only, the Sailing Heritage Society hosts free sails on Sundays, too. Drop by the H.C. Henry Pier on Lake Union to sign up.
(If you're more of a DIY type, check out the canoes at the U.W.'s Waterfront Activities Center. Rentals at the WAC are the region's biggest bargain -- for a mere $7.50 an hour, you can explore the wetlands and waterways around the arboretum. )