1) Without knowing how a lag in basics is translated into morale and fatigue impacts, or further, how those impacts affect a unit's ability to fight, I accept it by faith that falling short of consumables is a problem.
2) We spent a significant amount of time aggregating the number of ounces of water per soldier per day for drinking, washing, and brushing teeth required to sustain a highly mobile brigade that had to be deployed quickly by air transport, but still required tons of water simply so the troops could drink for the first three days after initial deployment.
1) In RL or in the game? Next to an enemy's bullet/shell/grenade, the threat of dehydration was the soldier's most evil enemy. On all sides, groups of soldiers had surrendered "just" due to the lack of water supplies, so lack of water should definetly have an impact on morale and fatigue, and especially on the physical ability to fight, in the game.
2) When troops were surrounded/cut off from supply deliveries, water was rationed immediately, so that it wasn't even allowed to be used for washing or brushing teeth, usually. Even the field manuals of the involved armies (all countries) of that era stressed to restrict the use of water to drinking and gave tips how to keep up a minimum of hygiene with minimal use of water and even without water. In contrast, in case of a lack of food supplies, modern field manuals, like the US FM 3-05.70 ("survival", 2002) stress to procure and drink more water, if the food supply level is low, for instance. A quote from the manual: "in any situation where food intake is low, drink 6-8 liters of water per day."
Also:
- "Conserve sweat, not water.
- Limit sweat-producing activities but drink water.
- Ration water.
- Until you find a suitable source, ration your sweat, not your water. Limit activity and heat gain or loss"
These are survival tips, of course, which may be impossible to follow in a combat zone, especially if surrounded, but they mark a different approach than the manuals during the war. Allied and German/Italian troops in the desert had different supply regimes, where basically both sides put a lot of effort into planning and distribution of water supplies, and it's actually quite impressive how the Italians (in the main) had managed to keep most of their troops supplied with water, despite their lack of (fast) transport vehicles, in North Africa. Afaik, the Germans denied handing over transport capacities for troop transport in quite some sectors, and - even worse - they denied to hand over trucks for water supply to the Italians in pretty most sectors, and they didn't handle much of the Italian water supply system, either.
The Brits in Arnhem assessed on the 20th of September that water supplies would last for one more day. Some houses in the outskirts had their own wells, but quite some water pipes in the city center near the brigde were probably destroyed, and I wouldn't be surprised if the Germans actually cut off the water pipelines supplying the city. Only 13% of the supply (supply drops) assigned to the British paras had actually reached them, and it can be assumed that the bulk rather consisted of ammunition supplies and food, than water.
Anyway,
3 days sounds realistic for urban or suburban areas or areas with natural water sources (small rivers, lakes, etc.), but probably less when cut off and forced to move and fight in say plain, wooded and steppe environments without any natural water sources. In the Ardennes, some cut off troops started to fill their cantinas with water from mudholes, quite some soldiers got diarrhea and desentery. US troops had chlorine-based Halazone tablets in their C-rations, which were used for (personal) water purification of unsafe water, but the short usable life of opened bottles and the limited amount carried by soldiers prevented continuous purification. I am not sure whether the Germans had similar tablets, but I tend to think that they didn't have those, as I know that Coys put quite some effort into securing water sources for the unit, by sending out what I would call water detachments or even single troopers to resupply platoons and groups.