My daily exercise for Socks involves a bikeride round trip of about 1.5 miles that includes a stretch up a hill and back down, which gets her pretty well exerted. After that part, which is near the middle, we stop for five minutes at a creek that's flowing into Dix Reservoir, so she can sniff, dig, do her business, drink, and since she's a lab, inevitably swim, and through all these, catch her breath before the trip home.
Dix Reservoir is a beautiful lake (pictured here) that is also the Barre town water supply. As such, it has very limited recreational use. There's one designated fishing area, plus another (near where Socks and I stop) that also allows fishing at certain times, and that's about it. No swimming, no watercraft.
While Socks isn't actually in the lake itself, she's in a creek feeding directly into it for those few minutes, so I have to be sure that this isn't in violation of the restrictions or their intent. Once, I even had one jerk (who was busily littering at the time) bitch at me for letting her swim in the reservoir -- but that he was a hypocritical jerk doesn't invalidate the question. On consideration, I'm fairly confident that letting her swim in the creek (or even the lake) violates neither the rule nor its intent, because there's two main reasons why letting people swim in the lake is a whole different thing from letting a dog swim in it.
First, a person dipped into a lake is going to release all kinds of contaminants. The person's body is covered in soaps, shampoos, skin creams, hair gel, bug spray, medicines, perfumes and colognes, tanning lotions, antiperspirant, and who knows what else. His clothes are full of even more cleaning chemicals. By contrast, the geese, beavers, deer, etc. that regularly visit the lake (you could hardly stop them) bring essentially nothing that isn't already in the lake -- what's on their fur is what's in the trees and soil and rain. A domesticated dog might not be quite as all-natural as an otter, but pretty close. Even if your dog gets bathed, it's probably once a month, not once a day, and probably involves a lot less soap that won't stick around as long, and no perfumes, tanning lotions, laundry detergent, or any of the rest. And while dogs might have a flea and tick prevention medicine or cream, that's also going to be far less than any human. A dog's impact on the water purification process will be a tiny fraction of a human's, and barely more than that of a few geese.
Second, letting a dog go for a five-minute swim a day is not going to cause a huge pack of other dogs to come cluttering the place up. But if you put up a sign allowing swimming, every sunny day you'll have a hundred humans, so that much greater impact will be multiplied by a huge number. Plus they'll be bringing their trash and failing to pack it out (or even put it in receptacles, if you provide them and send someone to empty them regularly). They'll be coming in cars which need to be parked, which means parking lot runoff and leaking car runoff, which will hurt the water purity more than everything else combined. They'll be dropping actual trash into the lake while swimming, and making plenty of messes. They'll be doing damage to water quality just through the act of erosion that far exceeds all my dog, despite her love of digging, can do in a lifetime.
So a person does many times more than a dog to the water, and allowing people would mean hundreds as many as allowing dogs, so all in all, the impact of one dog is trivial -- far less, I'd guess, than the difference caused by one extra rainfall, or one extra visit by a flock of passing geese -- while the impact of humans would be tremendous. I really don't think I need to deprive her of those five minutes of cooling off in the water. And I don't think I have to feel bad about it.