International Marine Underwriters (IMU) is a member of the OneBeacon Insurance Group. Headquartered in New York City, IMU is singularly focused on ocean marine insurance.

OneBeacon, a Bermuda-domiciled holding company, is one of the oldest property-casualty insurance companies in the United States. OneBeacon is rated "A" (Excellent) by A.M. Best, and is traded on the New York Stock Exchange under the symbol "OB."