My guess would be that water supports the body, so your weight doesn't strain your heart body as much, meaning it is easier to swim for overweight people than it is to run. Others say that because you become accustomed to the cooler temperatures of water, your body will hold on to a little bit of fat.
Of course, they could just be good swimmers. If they swim alot and have good technique they can quickly build up stamina and speed. Fat could just be from excess calories.