Earlier quoted context omitted.
And the ferries are heavily subsidized and very expensive to operate, and they pollute quite heavily. They don't compare remotely favorably to the subway along any axis except start-up cost (it's cheaper to buy a boat and build some piers than to build a subway tunnel).
except start-up cost Which is, well, hugely significant when you think about it.
Some people tried to make the math work for the L shutdown and even if you had a line of ferries just following each other as close as safely possible it wouldn’t be nearly enough. Subways are incredibly efficient at moving large numbers of people.