Green’s Dictionary of Slang

(W.I.) an African-born black person, so called by the Creoles, who were born in the West Indies.

1774		E. Long Hist. of Jamaica 410: The Creole Blacks [...] hold the Africans in the utmost contempt, stiling them, ‘salt-water Negroes’, and ‘Guiney birds’; but value themselves on their own pedigree.
1934		C.S. Johnson Shadow of the Plantation 22: [T]he ‘salt-water nigger.’ They were the freshly imported Negroes from Africa toward whom the Negro slaves felt a pronounced superiority.