Buttermilk Sound connects the Altamaha and South Altamaha Rivers south of where the Waterway crosses Altamaha Sound. As you will see by following the Chart View link below, there are already a number of Navigation Alerts due to shoaling through the area. The range markers Roger mentions begin at Mile 660 and take you very close to the missing marker, so caution is advised, especially at high tide.

16 Feb 2012
Green 211 is missing. N31 18.966 W81 21.761
Stay tight to the range in case the broken off stub is just below the surface.
Roger Long

2 Apr 2012 ‘“ The Coast Guard has placed Green Can #211 in the approximate location of missing Daymarker ‘˜G’ 211. At low tide the broken daymarker piling was not visible.
M/V Emerald Lady

Click Here To Open A Chart View Window, Zoomed To A “Navigation Alert” Position at Missing Marker #211