Crowds of men and women attired in the usual costumes! how curious you are to me!
He addresses the crowds of people on the boat, calling them "curious" (as in, "strange"). Keep an eye out for this word in the rest of the poem.
Our speaker must either be very perceptive or very quirky to find something noteworthy in an average crowd of commuters on their way home from work. He describes their work attire as "the usual costumes," which introduces a comparison with the theater. These normal people are really actors in disguises.
On the ferry-boats, the hundreds and hundreds that cross, returning home, are more curious to me than you suppose, And you that shall cross from shore to shore years hence, are more to me, and more in my meditations, than you might suppose.
Not only are the costumed passengers curious, but they are "more curious to me than you suppose," which, not to beat a dead horse, is something of a curious statement itself. Why would the other passengers have an opinion about whether this fellow staring into the water finds them strange, or interesting?
We think the speaker is protesting the anonymity of modern life, and the fact that you're not supposed to care about people you share a casual experience with.
Whitman wants his passengers to know: he does care! Not only about them, but even about the future passengers on the ferry, those who will "cross from shore to shore years hence."