BoatUS probably has the best coverage and service of anybody in the business. They were the only outfit that would cover a 20' outboard for trips to the Bahamas years ago; I was able to get a temporary rider for the duration of a trip that didn't cost much at all. They will often insist on a survey for an older boat and will recommend some good surveyors. The Corrective Action Teams they put together to help folks after hurricanes do some amazing work that always get rave reviews from their customers. Check out this link:
http://www.boatus.com/insurance/newslettersignup.asp