It avails not, neither time or place—instance avails not, I am with you, you men and women of a generation, or ever so many generations hence, I project myself—also I return—I am with you, and know how it is.
"Avails" is another one of those "important" words in the poem. It means to succeed in doing or accomplishing something; in this case, to succeed in separating people. Fortunately, "neither time nor place […] avails" in that task.
Put simply, there's just no escaping the speaker of this poem. He's connected to everyone of his generation and of future generations.
If you have no idea what he's talking about at this point, just think of Obi Wan Kenobi from Star Wars. There's some mysterious connection, like "the force," that connects all people, but only some are perceptive enough to realize it.
The speaker can also "project himself" into the future. So, even as the speaker is leaning on the deck, he has dispatched a part of himself to communicate a reassuring message to us future-readers.