Not all lifejackets will flip an unconscious person over. With so many posts stating this as a fact, I'm bringing this in.
Which type of life jacket was David wearing? I've seen a photo of David on a jet-ski and IMO, he is wearing a Type lll based on the appearance. These are the ones we use. They fit better and are less bulky around your arms. Which is fine when your skiing, tubing, etc.
From the link and there are examples of what each type looks like.
Type I - Off-Shore Life Jacket
The Type I PFD provides the most buoyancy and is best for open, rough or remote water, where rescue may be slow in coming. It is designed to turn most unconscious wearers face-up in the water. The Type I comes in both adult and child size.
Type II - Near-Shore Buoyant Vest
The Type II PFD is good for calm, inland water or where there is a good chance of quick rescue. T
his type will turn some, but not all, unconscious wearers face-up in the water. Less bulky than a Type I PFD, the Type II is the least expensive type of PFD. It is available in many sizes.
Type III Flotation Aid
The Type III PFD is good for calm, inland water or where there is a good chance of quick rescue. Designed to keep the wearer in a vertical position
, it may require the wearer to tilt their head back to avoid going face-down in the water and therefore is not recommended for extended survival in rough water. The Type III allows more freedom of movement for active water sports and is generally the most comfortable type for continuous wear. Float coats, fishing vests, and vests designed with features suitable for various sports activities are examples of this type of PFD.
Type IV Throwable Device
The Type IV PFD is an approved device which is designed to the thrown to a conscious person in the water. This device is not designed to be worn and must have at least 16.5 pounds of buoyancy. These usually take the shape of a boat cushion, life ring, or horseshoe device. These devices must be readily accessible during boat operation.
Type V - Special Use Device or Hybrid Inflatable
The Type V PFD is a special use device intended for a certain activity and may be carried instead of another PFD, but only if used in accordance with its label. Some of these devices provide hypothermia protection while others, such as a work vest, are intended for freedom of movement. A Type V may also take the form of hybrid inflatables such as float coats which combine inherently buoyant material with an inflatable bladder for extra lift. Type V PFD's must be worn when underway to be legal.
http://www.dnr.state.wi.us/org/es/enforcement/safety/pfd.htm