With training and when you are expecting to go into the sea yes obviously you can swim 20 miles across the English channel but all the cardiac arrests from cold water shock and general panic in the first 15 seconds bring the average down. The rule of 3 makes it easier to remember; they are approximate average numbers which are meant to drive the point home which is to wear a life jacket. We would prefer to find you shivering on a beach than face down in the sea.