I just got back from swimming a mile, fueled, as always, by my lowcarb diet. I've been swimming a mile a couple of times a week for about 30 years and I did not notice any change in my performance or energy when I switched to low carb eating two years ago (other than an increase in my energy level as I've dropped over 100 pounds). Granted, I don't swim particularly fast, but a mile is a good workout and if I needed lots of carbs to fuel it, I think I would notice.