The Seattle area has many rivers you can float down. The ones that come to mind are the Snoqualmie River, Cedar River and Green River.
It's an affordable way to enjoy water sports (with the other option involving getting a boat). All you need is a $5 to $15 floating tube and waterproof sunscreen. The tube doesn't need to be one of those heavy duty ones since you won't ever be going more than a couple miles per hour. One guy even had an air mattress.
We ended up in Fall City along the Snoqualmie River. The float took about 4 hours to go just a couple miles. We left a car near the bridge in town (the one right by Small Frys and Valero Gas Station), and drove up for about 5 minutes. A lot of fun, especially with a large group like ours.
Even if you're a terrible swimmer like me, it's worth trying and I never felt like anyone was in danger at any point.
