Ferries look cheap from a political capital standpoint, particularly if mayors are afraid of suburban motorists, I'll grant you that, but that's their flaw: they are a bad engineering solution dangled/touted by venal politicians to make it look like they're doing something useful and innovative.
I note that GE was smart enough to ask for a pedestrian bridge, not a ferry.
In both Wall St and Sydney the supertalls and subway come right to the water's edge.
In Boston we've actually got rules limiting height at the water's edge (eg Seaport), or short stuff that ain't going anywhere (North End)
Aquarium is 200' from the water, but has lousy vertical circulation and too much lowrise and parks
Lovejoy to NS Green/Orange is something like 600' in Boston weather
WTC SL is about 400'
In NYC Whitehall & South Ferry are 350' from the Staten Island Ferry's ramps, is near 100% covered and/or climate controlled and have great vertical circulation. (And a huge multimodal monopoly feeder hub on the Staten Island end)
The pattern I see is that old ferries work but
new ferries are just dangling A-to-B shuttles where neither A nor B has the necessary density. And we complain about single-door boarding on buses, but ferries have terrible station dwell characteristics docking & undocking.
2. You COULD create new ROW so you have more than 180 degrees of demand. AKA: A canal. Im not saying you should. Im just saying you could, and its probably cheaper than digging a subway.
The alternative to ferries is not "build a new subway" it is "deliver effective landside surface transit" (aka bus), but I doubt your canal would be cheaper (ROW acquisition costs would be an immediate killer--or else you'd have to 100% take a street (talk about political suicide) --and the problem with any canal is either that it has to be filled with freshwater (which takes a supply works) or seawater (which needs protection from storm surge).
Canals are also rendered impossible by underground utilities along any path. They are nonsense. It's why we stopped building them about 150 years ago.
Dirigibles! ROW is 100% Free! I've solved our mobility problem!