I didn't know that the Ferry was free until last summer when I set off to a Staten Island Yankees game (you can walk from the very nice ferry terminal to the stadium). I had only heard about it from my father and it had cost a nickel when he was a boy.
I have fallen in love with the ferry - hey, how can you knock a free boat ride through New York Harbor? Free sighting of Lady Liberty (so you can realize how little she is)? The answer is - you can't. Or you shouldn't. Seeing New York (all the boroughs) from the water is a great view of the city and we should all take advantage. But don't try this during rush hour or you will encounter some very angry commuters.