So here's the thing. It's about the "I" vs. the "we".
Shabine, known as Shabine "the patois for any red nigger" and nothing else, chooses the "we", leaves his island, and joins all the other Shabines to be lost at sea or wherever they might end up.
Coleman "Brutus" - traitor or defender of the Republic? sticks by the "I", giving up his past, chooses to forget.
Shabine can't forget.
Coleman is in the position where he can pass for a Jew, which backfires (silly people hating Jews)
Themes? Nationality, individuality, language.
Walcott chooses the native language for Shabine; Coleman has been brought up to respect words in the Shakespearian/Chaucer sense. This also backfires.
Shabine freely calls himself a "red nigger". Coleman is shocked to be labled thusly;
"But "nigger" -- him?" "he also discovered at Howard that he was a Negro as well"
He is aware that passing for a Jew will give him advantages (even his father; educated and proud, had to humble himself for work ("all he could do was meekly say "Yes, suh" - not his language, but the language forced upon him in order to keep the peace/his job).).

In short, white people in the 1920s, on the whole, sure did suck.