Lakes and rivers in Washington state are polluted AF. It's desperately sad.

Too many rich people building next to lakes and too many instances of sewage leaks and growth of coliform from feces.

Kind of wish lakes were public property and we didn't allow people to swallow up and poison the water by building so close because they want "lakefront property." Lakefronts should be public, imho.

Lake union I would never swim in, because of the major reasons you called out. It's heavily trafficked by planes and boats, and a lot of the boats don't care, or their rich people who are not paying full attention. I've seen people SUPing in there, at least you're more visible i guess. Idk if it's right by like, their dock they own then fine, because no one should be coming in, but I would never venture out very far. Way way way too busy of a waterway.

Alki Beach and Golden Gardens is where I'd go if you want to get in the water. I haven't heard any bacterial issues here in WA, but maybe do a news search on it? Those areas everyone expects people to be in the water and boats don't go near.

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.

Until you're a local, I'd be taking their advice.

If you want more "current" information because you don't trust the local "grapevine", contact the local tourist information centre, health department and/or environmental protection agency.