I regularly swim in Lake Washington, either at Madrona Park Beach or Seward Park (Bailey Peninsula). The peninsula allows for a great 2.5-.8 mile loop. I started out swimming only 1/2 mile in the lake last summer and worked my way up to 2.5-.8 miles. It's been a real accomplishment for myself, proving myself that you can do anything you put your mind to if you want it bad enough (Mom's motto).
I've also swam across Lake Washington in a race (total of 2.5 miles). Upcoming lake swims include the Fat Salmon swim (3.2 miles)!
The lake ends up being warm enough in July and August to not wear a wetsuit!