The USCG actually has no rule about it, it's a suggested HP rating, there is no requirement to follow it and the USCG has no authority to enforce it.

Any enforcement is done by the local authorities, and only if they have it as a law on the books. Many locations/regions have nothing to say about it.



This is directly from the Coast Guard, with the link below it.

They do not and cannot enforce over powering a boat, they can do something about a boat being operated in an unsafe manner though.

MAXIMUM HORSEPOWER – The maximum horsepower
information listed on the capacity label is a guide for
selecting an engine for a boat. It’s not a violation of Federal
Regulation to install or use a larger engine. Boaters should
check state regulations for restrictions. They should also
take a look at their insurance policy regarding horsepower.

http://www.uscg.mil/d1/prevention/Na...ing_Safety.PDF