We had a Hughes 500 owner who ran on red diesel. When we dropped the EDP fuel pump filter the water tidemark was 50% up the side of the filter. The 45 degree angle of the engine did not help as the water tended to migrate to the bottom of the filter bowl and stay there. I wondered how long it would be before the water level rose to the fuel outlet point.
In addition the fuel dye stain was there for all to see on the filter!!!!!! Image the effect of a steep nosedown which caused a slug of water like that to suddenly enter the fuel system.
Engine out warnings and a horrible silence from the hot bit at the back!!!!!
Commercial diesel can have a very high water content, storage and transport conditions are less stringent than aviation fuel.