I take a ferry is the summer to an island in Lake Superior. The captain reminds people that if you fall in, you have about 10-15 minutes before hypothermia sets in, even in the middle of July. The water is still in the low 40's then. And that has nothing to do with how good of a swimmer you are or if you're wearing a life vest. Year round, that lake more than even the others, seems designed to kill you.