If your boat has already been registered in the state and you just need to renew, you can do it by mail, go to any town clerk that does boat regs (they don't all), go to the main office in Concord, or one of the satellite branches.
If your boat has never been registered in the state you need to go to Concord or one of the satellite offices.
http://www.dmv.org/nh-new-hampshire/...gistration.php
http://www.nh.gov/safety/divisions/d...locations.html
Ken