A helicopter can go in a straight line and land on the apron, right next to a waiting aircraft. How long is a ferry going to take to get all the way around Brooklyn, through the bays and where is it going to dock when it gets there?! That'd take hours.
Not at all. A fast ferry from downtown NYC should take around 20 minutes to get to JFK. It would be easy for them to integrate a marine terminal there, just as they are going to do at LaGuardia airport. It’s a hell of a lot more practical than a helicopter service.
Such ferries do exist (hydrofoil or hovercraft) but sailing through a congested channel at 60 mph would be wildly dangerous. I can't imagine the Coast Guard or port authorities would ever allow it.
Yes this would be a challenge. It may not be able to operate at full speed for the entire duration of the trip, but significant portions it probably could.
A helicopter can go in a straight line and land on the apron, right next to a waiting aircraft. How long is a ferry going to take to get all the way around Brooklyn, through the bays and where is it going to dock when it gets there?! That'd take hours.