I believe the correct spelling is Devereaux, and not Deveau. Both are old French Huguenot surnames, but the Deveau family lived pretty much along the lowere Santee River, to the north and northeast of Charleston, while the Devereaux/Devereau family lived more toward Beaufort.
Coosawhahatchie was -- and is -- the name of a Low Country River whichis located in present day Jasper County. The town of Coosawhatchie (and where the bridge was located) was on present day U.S. 17 just to the southwest of the town of Yamassee. It was the location of the railroad bridge between Savannaah and Charleston over the Coosawhatchie River. Devereaux Neck was in the same general vicinity.
Hope this helps, a little.