Boat insurance is available through almost all of the large national insurance companies (Allstate, Geico, Progressive, State Farm, etc.) as well as through many small local and independent firms/agencies. Your local marina, lender, or boat broker should also be able to point you in the direction of some resources for finding boat insurance.
Lastly, consider getting a quote from the Boat Owners Association of the United States (BoatU.S.). They've been offering boater specific insurance products and services for several decades and are an excellent resource for information about the industry.
The following are unpaid links to boat insurance and quotes:
Large Insurance Companies Offering Boat Policies
Allstate Boat Insurance
Geico Boat Insurance
Nationwide Boat Insurance
Progressive Boat Insurance
State Farm Boat Insurance
Companies That Specialize in Boat Insurance
BoatU.S. Boat Insurance
United Marine Underwriters Boat Insurance
Marks Marine Insurance
Seaworthy Marine Insurance
International Marine Insurance Services
Markel Boat Insurance