Now I won't even begin to suggest I'm an expert on the subject here, but to your point LIforrelaxin...
I'll generalize here and say that anything that is capable of making noise would thus be measurable at any particular distance which is what laws stipulate depending on what it is you're talking about. Perception of how loud they are can be relative, and in some cases depending on the situation actually may appear to be magnified. Case in point, look at the hat shell in Boston or my favorite Red Rocks in Colorado. These places one made made, the other God made have acoustical properties that appear to magnify sound. So while from a pure db perspective the noise levels may be considered acceptable in certain cases they could "appear" to be far worse. That is something that the law can't address so it is what it is and certainly not anything the operator of a boat can control. Good bad or indifferent, it's my best stab at a "very" scientific observation based on absolutely no scientific knowledge or theory in said discipline.
.... it just sounds good to me!