Here is a link to the Rules for NH Bodies of Water (see below) and the general rule of thumb is 75 feet from shore, but Marine Patrol will tell you when they come over to your anchored boat that you must be at least 150 feet from the shore line.
http://www.state.nh.us/safety/ss/bodies.html
I think we need to hear from our MP's or other state officials out there for some clear definition.