In New Hampshire the open container law concerning alcoholic adult beverages in a motor vehicle does not include water vessels or a boat. Boating and drinking an adult beverage is legal. What's not legal is boating with a blood alcohol level higher than "0.08 higher by blood, breath or urine. Or between .03 and .08 along with other evidence."
So it is legal to be driving a motorboat, or paddling a canoe with an alcoholic beverage in one hand and the steering wheel or canoe paddle in the other hand, as long as your blood alcohol level is lower than 0.08, or eight percent.