It's a myth that all dogs know how to swim naturally, or that they can swim for extended time without floatation. Just like humans, dogs can get fatigued, especially in cold water. There's simply no good reason to risk going out on the water without a sound floatation device for everyone including your dog. For best fit, measure your dog around the widest part of his or her rib cage (girth). Length (from back of neck to base of tail) is good to know but not as important as girth.