It's not a dumb question at all.
There is no official maximum horsepower above a length where a placard is not required. Naval architects for the most part will not design a boat with an unsafe amount of horsepower.
There is no law that a naval architect cannot design a 3000 horsepower offshore racer
capable of traveling over 100 MPH where it is relatively easy to kill oneself.
For smaller boats, under 20 feet in length, there is a placard which lists the maximum allowable horsepower, maximum weight and maximum number or passengers. These maximums are calculated by a formula that is federal law to help keep people safe.