The certificate you receive for passing the NH Online course and test will be good for life! The info on their web site was recently updated to announce the "new law".
From the NH Official Boater Ed site
New Hampshire Law Change Goes Into Effect January 1, 2007
Until January 1, 2007, all the current boating safety education rules remain in effect.
Passing the Boat New Hampshire online course is sufficient to get your boating safety certification until year-end 2006.
Beginning in 2007, students must take a proctored, in-person test in order to become certified. So, effective January 1, 2007, an online certification exam will no longer be offered.
Once an individual has become certified, they have fulfilled the requirements of the law. There is no legal obligation to take a boating safety course again. IT IS A ONCE IN A LIFETIME REQUIREMENT.
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The way I read this it seems clear that once properly certified (even via the NH on-line course and test) you have satisfied your NH requirement for life.
I would not mind a requirement for a routine refresher course every 5 or 10 years but this is the law as it is. So, I expect that the NH on-line course and certificate is good until I don't need one anymore.
BTW: MP told me that my old USCGA Safe Boating course certificate was NOT acceptable because it was too old. They mentioned something about earlier than 1980 (they didn't give me an exact year) did not cover new enough information. I took that course before I was old enough to drive a car. When in doubt, check it out and call MP to be sure.
Safe Boating to all.