From the USCG:
Personal Flotation Devices (PFDs): By law you must have one PFD (Type I, II, III ) on board for each person aboard. A type V PFD may be counted it is is being worn. If your boat is 16 feet or more, you must also have a throwable PFD (Type IV).

Type I PFD: Off-shore life jacket. Provides a minimum of 22 lbs of buoyancy. Recommended for off-shore boating where delayed rescue may take place. A Type I PFD will turn most unconscious persons face-up.

Type II PFD: Near-shore life jacket. Provides a minimum of 15.5 lbs of buoyancy. Recommended for coastal or lake boating where prompt rescue is likely. A Type II PFD will turn most unconscious persons face-up.

Type III PFD: Flotation Aids such as ski vests are designed for comfort and ease of motion.


Type IV PFD: Throwable Device: For example: Life ring, floatable cushions.
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The other types of lifevests other than Type I trade off face-righting ability for comfort and mobility. They are only reccomended for coastal and inland boating. If you not comfortable with that then make sure you get a Type I or Type II vest. Also make sure your trap harness does not have any extra padding on the bottom that may cause unwanted flotation.