I actually found the CFR for federal law but I'm not going to post the whole section - it's long and it's complicated:

http://www.gpo.gov/fdsys/pkg/CFR-2001-title33-vol2/pdf/CFR-2001-title33-vol2-sec183-53.pdf

Here's the Reader's Digest summary:

§183.53 Horsepower capacity.
The maximum horsepower capacity marked on a boat must not exceed the horsepower capacity determined by the computation method discussed in paragraph (a) of this section, or for certain qualifying boats, the performance test method discussed in paragraph (b) of this section.