I grew up on the other side of the Saelish Sea (Puget Sound). I’d not recommend must places in King, Pierce, or Snohomish counties (or basically any major urban area). I’d also not recommend salt water anywhere but the coast. If you want to go lake swimming, there are good spots on the Olympic peninsula (ex Lake Cushman), places like Whatcom Falls in the North, or Icicle Creek out towards Leavenworth.
Basically, anywhere in the Seattle metro area will be questionable, IMO.