Try heating the barrel. The water should drop out to the bottom, and the diesel should stay on the top.
...Yeah...what he said.Heating and settling your waste diesel fuel are good ideas. You would want to be careful how and where you heat a barrel of diesel fuel, because the whole thing could blow up in you. And, make sure the lid is off so the pressure in the drum does not build up. I would put the drum in the sun on a sheet of ply wood and place a clear plastic 55 gallon "contractor" bag over the barrel and let the sun heat it. The temp should not exceed 150F. The water should separate out in a day or two of cycling the drum temperature.