Do you know what regiment your ancestor was in?
Coosawatchie was a town located between Savannah and Charleston. R.E. Lee was in command of this area from roughly the fall of 1861 until the spring of 1862, and was in charge of a small but mobile force of a handful of Confederate regiments whose job it was to protect the strategically important Charleston & Savannah Railroad from Union troops positioned offshore to potentially take it.
The Port Royal expedition by the Federals in Nov. 1861 gave the Yanks a toehold on the outer regions of this area. The Federals attempted to move inland on Jan. 1, 1862, but were met with Confederate resistance by some of the CS forces mentioned above and were unsuccessful in making a further advance.
The link below describes some of the action of Jan 1, 1862, and has a link to the Nov. 1861 expedition.
If you could tell me the regiment your ancestor was in, I could possibly tell you more specific information about its whereabouts in the area, as I have done a fair amount of research on this subject in years past.