In my country, very similar, except that it happend in a fresh water environment with a big freshwater lake in the middle and that the coal deposits were formed during the Late Carboniferous (few lenses) , the Permian (most of the coal) and Early Triassic (big, "coaly" deposits such as the Waterberg coal).
I think that in most places the waters are fresh to brackish. These are mostly swamps and if no one is intercepting the surface and groundwater flow they should be more or less fresh. The Everglades in Florida would be an example where the flows are of fresh water to the south. In that case, however, we are getting salt water incursion into the water table.
This goes back to the delta question where we have fresh water flowing into the ocean to some degree. The Amazon supposedly forms brackish zones far into the Atlantic Ocean, particularly at the surface. I haven't tried to drink it...
I think someone mentioned the Amazon delta. The curious thing is that it is so small...