Any of the large rivers (of which you are near one) or the coast could technically be used as a 'flyway' line of navigation for migrating ducks. Granted, no where in NH will be like in the videos, no matter where you are.
I've had mornings where I see hundreds in the morning flight and I've seen morning flights where I'd like to think my eyes were closed because I didn't see a single one.
It is all dependent on migration, pressured local ducks, weather, location (big factor to numbers and species)-- so it is really hard or technically impossible to answer this question. If you want to have reasonable expectations, expect to see a pair and that way you can be excited when you see a lot and not as bummed when it is a slow morning.