Interesting debate on the possibility of constitutional ramifications playing in to the speed limit proposal.
However just as there is no constitutional “right” to operate a motor vehicle on a public way there is also no constitutional right to operate a watercraft on publicly owned and controlled waterways. The ability to operate a motor vehicle or a boat upon a way is a privilege controlled by the government. And the government has the ability to grant, revoke and heavily regulate those privilege, much to the chagrin of more than a few!