I could be wrong, but I believe a misdemeanor plea to Careless and Negligent Operation of boats carries a $500 fine. Could have been the reason why they didn't pursue a BWI. I imagine Marine Patrol does a lot of court work year round so just because you aren't hearing about it in the papers doesnt mean that its not happening...
For instance, the website
www.byebyedwi.com (lets not get in to ethics here) has some examples of recent BWI cases in N.H. Take it for whatever its worth, and just remember that the webmaster is only posting cases that they won...
It seems that any DWI case on the road or water is pretty hard to prove these days.