Looks like most regs are related to motor noise levels. (Had me curious. Needed to look it up.)
http://www.pca.state.mn.us/sites/default/files/p-gen6-01.pdf
How loud can a boat be in Minnesota?
(2) for marine engines or motorboats manufactured on or after January 1, 1982, a noise level of 82 decibels on the A scale measured at a distance of 50 feet from the motorboat or equivalent noise levels at other distances as specified by the commissioner in a pass-by test or 84 decibels on the A scale measured at idle.
BTW, there are plenty of OTC sub woofer and driver packages out there. No need to build one. Like this one for example. Sub woofer with amp in easy to install package.
-J.