A PFD greatly reduces the chance of sudden drowning due to cold shock and swimming failure. With few exceptions, cold shock causes people to immediately lose control of their breathing. As a result, many of them suddenly drown – even though they can swim.

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Cold water drowning can occur instantly if cold shock causes a person to gasp while their mouth is submerged. That particular phenomenon used to be called "Sudden Disappearance Syndrome", and it's worth emphasizing that you don't suddenly disappear and sink to the bottom if you are wearing a properly secured PFD.

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Drowning can also occur during the first several minutes of cold shock due to swimming failure or inhaling water as a result of wave splash. If you aren’t wearing a PFD, you will drown as soon as you can no longer swim or tread water.