A PFD (class I, II, III, & V) is a wearable device that will provide the "extra lift" to keep you afloat until help arrives.
Always make sure that your PFD is properly fitted for you and make sure it's U.S. Coast Guard approved.
The U.S. Coast Guard says that you need at least one wearable device for each person on board or being towed. Boats 16 feet in length or longer must have a type IV throwable device on board.
REMEMBER, LIFE JACKETS WON'T WORK IF YOU DON'T WEAR THEM.