The beach at Matthews Beach Park (5100 NE 93rd St.) is currently closed for wading/swimming after water quality monitoring results showed high bacteria levels. King County will be retesting the water and the beach will be closed through at least the weekend. More information and updates available here: https://green2.kingcounty.gov/swimbeach/
The north and south beaches at Discovery Park are currently closed due to a sewage spill at the King County West Point Treatment Plant. Updates on the beach closure are available here: https://ecology.wa.gov/Water-Shorelines/Water-quality/Saltwater/BEACH-program
Seattle Parks and Recreation strongly recommends swimming only where lifeguards are present. See a list of lifeguarded beaches here: www.seattle.gov/parks/find/swimming-beaches