I too have an ancestor named William Dewitt (Dewett) from SC. The birthdate I've been given is 1798. He died in Neshoba Co, MS (1867). He married 1) Mc Afee (two children Thomas W. Dewett and Margaret Dewett); 2) Married a Beggs in AL; 3rd wife) Elizabeth Castles in AL. They had 9 children together and lived in MS. Is this one possibly related to your William? Name has since changed from Dewett, Dewitt, Duett, to Duette for my group. The majority still "Duett". I hdn't seen the DeWhitt spelling yet, but maybe so.