I saw what looked like a boy scout troop swimming next to a couple canoes. Can't say it really bothers me, and I doubt it's enforced at all. They were swimming for about half an hour right around the corner from Fountainhead. It strikes me as the kind of thing a park might have to prohibit for liability reasons, not necessarily because anyone cares.
That said, I wouldn't go swimming there. Not because of any park rules, mind you. Mostly because lake swimming--and the accompanying mud, smell, salt, and sunburns stopped being fun about 20 years ago.