Final answer:
Throwable PFDs are required on boats 16 feet in length or longer, according to United States Coast Guard regulations, and must be immediately available for use in case of someone going overboard.
Step-by-step explanation:
According to United States Coast Guard (USCG) regulations, throwable PFDs (Personal Flotation Devices) are required on boats 16 feet in length or longer. Operators of these boats must ensure that at least one USCG-approved throwable PFD is immediately available for use. Examples of throwable PFDs include devices like cushion-type buoyant devices, ring buoys, and horseshoe buoys. These are not meant to be worn but are designed to be thrown to someone in the water to provide buoyancy until rescue can be accomplished. It is important to note that personal watercraft and kayaks are not required to carry a throwable PFD, but it is still recommended for safety.