You can check water conditions (for example, if a beach is closed) at recreational swimming areas in King County via this Washington State Department of Health web site.
Seattle is host to several Polar Bear Dips in January. One is at Mathews Beach January 1st. BFC's Polar Bare Dip is generally the saturday closest to Ben Franklin's Birthday (Jan 17th) and is clothing-optional. The event moves every year. See BFC's Polar Bare Dip page.
The Sun Lover's Under Gray Skies (SLUGS) also offers family-friendly clothes free swim nights and Russian Urban Spa/Sauna/Steam room nights during the winter.
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