National Marine Underwriters (NMU) is a member of the OneBeacon Insurance Group. Headquartered in Annapolis MD, NMU is singularly focused on private pleasure craft insurance.

OneBeacon Insurance Group offers a range of specialty and segmented commercial and personal insurance products sold primarily through select independent agents. As one of the oldest property and casualty insurers in the United States, OneBeacon traces its roots to 1831 and the Potomac Fire Insurance Company. Today, OneBeacon's specialty insurance products are available countrywide, and commercial and personal lines are offered in select geographic territories. OneBeacon's principal business office is in Boston, Massachusetts. The company is publicly traded on the New York Stock Exchange under the symbol "OB".