I have always borne in mind an axiom (picked from the history of military medicine) that an "ancient army was a walking health hazard", to itself and whomsoever it aggressed. This is not to say that this was unknown by our ancestors , but that those who failed to plan for medical eventualities would quickly have enteric problems in abundance (and contagion by unknown viral agents in remote localities) . The Macedonians must have been tough customers, probably due to all that outrageous wine imbibing they did, instead of using suspect water.
Here is a quote regarding the poppy "It is unexcelled as a hypnotic and sedative, and is frequently administered to relieve pain and calm excitement. For its astringent properties, it is employed in diarrhoea and dysentery, and on account of its expectorant, diaphoretic, sedative and antispasmodic properties, in certain forms of cough, etc." Dysentry being the first thing that came to mind .
Water from gravel would at least have the virtue of some natural filtrattion.