There are three PFDs that would be considered readily accessible on a boat: Type I, II, and III.
Type I is the most buoyant and is designed for offshore use. It is the largest and heaviest of the three types of PFDs.
Type II is less buoyant than a Type I, but more so than a Type III. It is designed for nearshore use and typically has more flotation material than a Type III.
Type III is the least buoyant of the three types of PFDs, but it is still required to have at least 15.5lbs (7kg) of flotation material. This type of PFD is intended for use in calm water where there is little chance of being submerged.