Six knots in NY harbor or on the Hudson? No way, not unless they sneaked in turbines to boost the flow. Maybe at flood in the Hell Gate, but not on the Hudson. Four knots, sometimes, briefly, tops. 2-3 more typical and 2 most likely, which is one reason the harbor silts up so nicely.
So Lincoln Harbor and Liberty Landing are both accepting "occupied" boats? Nice to know someone still is. 200/ft may seem high--but even NJ rents on the Hudson aren't cheap, you can figure a small studio apartment for $1500/month is considered a bargain, that's the comparison point. That would price out against about a 90' boat if I haven't messed up my math again. Except of course the apartment sometimes includes heat and hot water at that price.