Understand Life Vests and PFD's personal floatation devices

People need to understand their personal PFD's

The people hardest to float are those with compact, dense bodies. These tend to be people with athletic body builds, with a lot of bone and muscle mass, and not much fat. Fat is not as dense as muscle and bone, so people who are overweight can actually be easier to float than someone who is much smaller and leaner. Heavy people do not need a higher buoyancy PFD because of their weight......

Most require only about 11 pounds (50 Newtons) of extra buoyancy to keep their head out of water. That is why a PFD with just 15.5 pounds (70 Newtons) of buoyancy can provide adequate flotation for an adult -- even a very large person. PFDs with 22 to 34 pounds (100 to 155 Newtons) can provide superior performance.
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Modifying a PFD, such as (permanently) adding additional body strap webbing, voids the device's approval, no matter who does the modification. However, for a person with a chest size over 130 cm (52 inches), using an adult universal PFD with a clip-on body strap extension does not void the approval.

THE BIBLE of PFD's - US Coast Guard - Last Modified 7/31/2008
http://www.uscg.mil/hq/cg5/cg5214/pfdselection.asp

BUOYANCY: Most adults only need an extra seven to twelve pounds of buoyancy to keep their heads above water. A PFD can give that "extra lift," and it's made to keep you floating until help comes.

Your weight isn't the only factor in finding out how much "extra lift" you need in water. Body fat, lung size, clothing, and whether the water is rough or calm, all play a part.