I've run an awful lot of water through PT6's and never seen a cough. Of course I'm an awful chicken too and always used the seperator as per the book (lots of flying Caravan's and Pilatus PC12's as well as B200 and 1900)! We used to use the separators all the time on dirt runways as well and that made huge differences in the life of the engines from times previous to that policy. They really work well at keeping the merde out of the engines, so use 'em!