hate to get all Joe scientist on you here but...
Water and sound don't mix well. Because water is extremely dense compared to air, sound travels 5 times faster under water and 10 times louder. Screaming on your boat has little effect on fish but the sound of something being dropped in the boat or in the water has a dramatic affect. Same goes for a gas engine. And is why many people crawl on their hands and knees to native trout streams.
"We're going to take things away from you on behalf of the common good"
Sen. Hillary Clinton - Speech at Democratic Fundraiser, June 2004