This is called hypothermia. If your body temperature goes too low, you may pass out and then drown. Even if you're wearing a PFD, your body can cool down 25 times faster in cold water than in air. Water temperature, body size, amount of body fat, and movement in the water all play a part in cold water survival. Small people cool faster than large people.
Children cool faster than adults.
But PFDs can still help you stay alive longer in cold water. They let you float without using energy and they protect part of your body from cold water. In cold water, they're better than vests because they cover more of your body . When you're in cold water, don't swim unless you can reach a nearby boat, fellow survivor, or floating object. Even good swimmers drown while swimming in cold water.