While Skip may link to the site showing 13 "substations" they may not be able to help you.
I personally contacted Concord via telephone and asked that very question when I first tried to register a trailer and I was told I could only register my trailer via town hall and that the boat could be registered via town hall, the state or via some marinas.
I also asked about those substations that Skip mentions and was told it was for drivers license renewal or renewal of registration but I could not use them for new registrations.
So either a state employee got it wrong, or...
Regarding the boating safety certificate...I don't care how many times that Skip or anyone else claims it's a license. It is not a license. It does not expire and it is not renewable. It is a lifetime certificate of accomplishment.
While your boating priviledge may be recinded in NH for violating the law, NH can not recind your certificate of accomplishment of passing a safe boating class especially if it was not issued in NH to begin with!
Even the NH regulation states it is a lifetime certificate.