Jake,
Yes, the throwable is a USCG requirement for boats over 16 feet. They describe its use as follows:
[color:"green"]"A TYPE IV PFD, THROWABLE DEVICE is intended for calm, inland water with heavy boat traffic, where help is always present. It is designed to be thrown to a person in the water and grasped and held by the user until rescued—It is not designed to be worn. Type IV devices include buoyant cushions, ring buoys, and horseshoe buoys. There are no inflatable type IV devices."[/color]
When I got to thinking about how we carry the device simply to meet a requirement designed for other types of craft, I began to wonder if it could be made useful.