There are five types of PFD approved by the U.S. Coast Guard for safety on the water.
Type I PFD - Offshore Life Jacket
The model best-suited to open and rough waters, a type I PFD provides more buoyancy than any other type. The design of a type I PFD allows it to turn most unconscious wearers into a face-up position with their head out of the water. This type requires a minimum adult buoyancy of 22 pounds, and because of its bulk it is generally not comfortable to wear when not on the water. These PFDs are only used in an emergency. They are typically jacket-shaped but sleeveless, and usually have multiple ties and belts for closure.
Type II PFD - Near-shore Buoyancy Vest
Familiar to anyone who has rented a canoe or other pleasure craft, these are the bright orange vests also seen on water taxis and the like. They are a reduced version of the type I PFD, and provide a minimum 15.5 pound buoyancy. They will usually turn the face of an unconscious person out of the water, but are not as dependable as type I PFDs for this task. Type II PFDs are used near shore where a quick rescue is likely. They usually have one belt and one tie.
Type III PFD - Flotation Aid
Most popular with canoeists and kayakers, a type III PFD is best for conscious wearers who can keep their own faces out of the water. The minimum buoyancy is 15.5 pounds, but some designs have higher buoyancy (frequently 17 pounds). Type III PFDs are usually jacket-style and may have pockets, lashing hooks, tow belts, and other functions that enhance their application. They typically fit the wearer closely, and many zip or have buckles to close.
Type IV PFD - Throwable Device
Throwable PFDs are designed for areas where there is constant boat traffic and rescue is immediate. They are commonly ring-shaped, but horseshoe and cushion type IV PFDs are also made. These are only a backup measure and should generally be thrown by someone with experience, as it is difficult to aim well, especially in rougher water. A cushion-style PFD has a buoyancy of 18 pounds, while a ring-style has a buoyancy of 16.5 pounds.
Type V PFD - Special Purpose
These PFDs are intended for specific uses, such as whitewater activities or boardsailing. Their turning performance (keeping an unconscious person face-up) is rated according to PFD types I, II, and III; some may also require that they are worn in order to be effective. Type V PFDs come in a variety of styles, from full-body suits to work vests. Some have a safety harness and some provide protection against hypothermia.


According to the Coast Guard, all recreational boats must carry one wearable PFD (Types I, II, III, and V) per person on board. Boats over sixteen feet in length are also required to carry a throwable (Type IV) PFD, but canoes and kayaks over 16' are exempt from this rule. PFDs must be approved by the Coast Guard (all PFDs will carry a label indicating they are USCG-approved; this label should never be removed) and they must also be in good condition, as well as being an appropriate size for the wearer. (Child-size PFDs have different buoyancy requirements than adult PFDs.) It is extremely important that wearable PFDs, if not actually on their designated person, be at least readily accessible. If an emergency arises, they must be situated in such a way that they can be easily put on.

Inflatable PFDs are sometimes considered more comfortable to wear, but they require proper care. They must have a full cylinder and indicators must read green. There are no Type IV inflatable PFDs, and they are sized only for adults. Type I and II inflatables have a buoyancy of 34 pounds, and type IIIs have a buoyancy of 22.5 pounds. There are also type V inflatable models, but their buoyancy ranges from 22.5 to 34 pounds.

Laws about PFD use vary from state to state. The only federal laws related to PFD use indicate that they are not required on racing kayaks, racing canoes, rowing sculls, or racing shells. Many states do require PFDs for towed activities such as water skiing, as well as when operating personal watercraft, during whitewater activities, and when sailboarding (even though sailboards are not technically "boats" according to federal law).

On a final, personal, non-factual note: Please always wear a PFD when you are on the water. Properly-fitted PFDs are perfectly comfortable and do not restrict your movement. No matter how much experience you have in whatever sport brings you onto the water, you can always drown if you don't have a PFD. Just keeping it handy in the boat isn't good enough, either: if you capsize, then your PFD can easily float away before you can get near it, and putting on a wearable PFD while in the water is nearly impossible. They float, you don't! Please wear your PFD!

Sources
http://www.uscgboating.org/safety/fed_reqs/equ_pfd.htm
http://www.physsportsmed.com/issues/1996/05_96/pfd.htm

Personal experience

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