Just a couple of more things. There was a record loss (a courthouse fire I think) in the parish in the late 1800's so there are not many records prior to 1897 (including marriages). There are other resources (Freedman's Bureau, Catholic Diocese, if they were catholic) for marriage records but those records are very hit or miss. Also, if this is an important area of research for you there is a book that I found to be pretty good background reading: "History of Washington Parish, Louisiana, 1798-1992" by E. Russ Williams. You could probably get the book thru inter-library loan if you're interested.
Good luck !