In simple terms an underwriter essentially promises to cover the costs if it all goes horribly wrong. In commercial insurance for example, Lloyds underwriters are individuals who put up their own money to cover possible insurance claims. Obviously they make money from the premiums paid by the insuree, but if the boat sinks or the building collapses or the car is written off, then the underwriters are gonna be out of pocket cos they have to replace the boat or building or car. I don't know whether an insurance company would be able to help you.