Lifejackets were designed to keep a person on their back. This would keep their face out of the water if they are unconscious allowing them to breathe.
Lifejackets come in two sizes. The first is for people who weigh more than 40 kg (88 lbs) and the second is for people who weigh less than 40 kg (88 lbs).
In order for the lifejacket to work properly, you need to wear it somewhat loosely. This will allow some water to pass between your body and the lifejacket. This will allow the lifejacket will turn you onto your back and keep you afloat.
Pros: keep you on your back and your face out of the water.