How difficult and expensive is it to muffle the sound of the motor on a boat?

I road one of the transportation boats at Disney a few days ago and the engine is so LOUD. I chose to sit right by the noise, because I wanted to be outside and the rest of the seating is inside, but it was so annoying. I was just curious why they don’t bother making that ride more pleasant. I wore my earplugs, but still, very loud.