From this website:
http://des.state.nh.us/factsheets/bb/bb-9.htm
"Do not bathe, shampoo, or wash boats, pets, or other objects in the lake with soap or phosphorus-containing detergents. Do not wash automobiles near lakes where the detergent can run into the water."
I'm pretty sure it's illegal to allow rinsate from boat washing into the lake, I can't find the law however. Common sense would tell you that rinsing soap into the lake is a bad idea.