It's pretty impressive (and not always widely known) how much cold water impairs swimming ability. There are some good videos on this site - coldwatersafety.org - of Coast Guard volunteers trying to swim in 45deg water. They lose motor function before making it 20m or so. Only one is able to make the short distance to shore, and he is one of the largest guys and a professional rescue swimmer. A 6km swim in 45F water for someone that is not wearing protective clothing or well acclimated to cold water is not realistic.