There are three places I know of where you can go skinny-dipping in Seattle. All three are beaches within park areas, Denny Blaine and Magnuson Parks on Lake Washington, and Discovery Beach on Puget Sound. I've only been to Denny Blaine Park, so I can only recommend that one for now. It's a small beach on the east side of seattle close to several bus lines, so it's easy to get to. It may be small, but it gets a lot of people on weekends and no one is bothered by nudity. Even families with kids don't bat an eye. It's also popular with the lesbian community in Seattle, many of whom like to go topless at Denny Blaine (I talked to a few of them). I highly recommend this one, as Lake Washington is significantly warmer than the sound, though the waves can get large on windy days (Lake Washington is very big). If anyone is there next summer, they should give it a try.