N.H. Rev. Stat. Ann. § 270:30-a

No child 12 years of age or under shall operate, control, or be transported on the deck of a boat or vessel which is underway upon the public waters of the state unless said child is wearing a personal flotation device of a type approved by the United States Coast Guard; provided, however, boats, vessels and ships with continuous side rails enclosing the perimeter of the boat, vessel, or ship, 3 feet or more in height and enclosed between the deck and the top of the railing in a way that would reasonably prevent passage of a small child are exempted from the provisions of this section.