An airboat is also louder than loud. Like really fucking loud. Like the engine is right behind your head loud. Unless we can run it flat out and make the trip short, we're probably better off on foot--probably up the New York side of the river through Yonkers and on north, since the Jersey side of the river is surely impassably hellish. I wonder if the Tappan Zee bridge is still up.
If we're going to make noise and be obvious, why stop at airboats? Why not look for an aircraft of some kind?
If we want a boat, though, we might could talk to the Mirelurk hunters. Or the Ferrymen. Or even the NYPD--I bet they've got a boat or two, or at least know of a salvageable specimen (though we probably don't have time for extensive repairs).
Incidentally, the trip looks like it's between 55 and 65 miles, depending on how we do it and possible detours. On foot, that's either a hard-pushing three days or a more manageable four. Do we feel like we've got three or four days (assuming the BoS will fly back in 30 minutes with vertibirds full of Empire-protecting zealots)?