"Crossing Brooklyn Ferry" begins with a cry of surprise and delight:
Flood-tide below me! I see you face to face!
The speaker, readers gather, must be standing on the Brooklyn ferry itself, a boat that takes people across the East River, back and forth between Brooklyn and Manhattan. Below him, he sees a "flood-tide"—that is, high tide on the river—and personifies it, greeting it like a friend, "face to face."
This moment suggests that the speaker is recognizing the river, seeing it fresh, noticing its character as if for the first time. The same thing happens between him and the sky a moment later: the "clouds" and the setting "sun" also appear to him "face to face." It's as if he's having a sudden, powerful encounter with the world around him. And his exuberant exclamation points in the first line suggest that this encounter might be both beautiful and startling.
In other words, on his everyday journey across the river, this speaker seems to have had an epiphany—a sudden new insight into the nature of the world. And that epiphany will have a lot to do both with seeing the world "face to face," and with seeing other people face to face—even if those people don't, strictly speaking, exist yet. This poem will trace the speaker's joyful realization that he's connected to everyone who ever was or ever will be, across space and time.