NH does not title boats. All you need to register your boat in NH is proof of ownership. A Bill of Sale is adequate proof. There is no specific Bill of Sale form that is required. At a minimum, I would suggest that the Bill of Sale reference the hull ID#. A registration form (RDMV 110), showing the required information, can be found here:
http://www.nh.gov/safety/divisions/dmv/forms.html
A boat trailer DOES require a title. Information applicable to trailer titling/registration can be found at the same website listed above.