Dewatering has two meanings with regards to coal mining.
The general mining sense of dewatering is the process of removing water from a mining site. There are many side effects of dewatering including a decrease in surface water in the surrounding area, decrease in the level of aquifers (and corresponding loss of shallow wells), loss of aquifers, as well as heaving and subsistence of the ground above the water loss.
For a technical paper on measuring the effects of dewatering see http://www.isprs.org/proceedings%2FXXXVII%2Fcongress%2F7_pdf%2F2_WG-VII-2%2F23.pdf
A second meaning of dewatering refers to a method where water is removed from coal after mining to reduce the mass before transport, or where coal tailings have water removed from them to try and reduce water usage.