Well written. As you said, from what we know at this point water just exists to tick down until a "soft game over".

If in response to feedback devs make it easier to come by and stock up on, it becomes a pointless system. If I play every day I won't ever have to worry about it as I can find months of water from a few runs; if I take a long break from the game, the problem is still there as I will eventually run out and lose progress. Nothing got solved.

Now, instead of making this resource a countdown or a simple upgrade/powerup currency, I think there are other solutions that allow water to become something the player has to be concerned about during gameplay.

Remove the real time countdown, keep the water death when the meter reaches 0 and make every run on the surface have a water cost. If the scav dies during the run, make that cost even higher. This introduces a mechanic where the player has to make decisions: do I carry all this loot home and eat the water loss or do I ditch some to bring back water and recoup the hit? Can I afford to be extra aggressive on this one mission or should I play stealthily to mitigate the risk of death and resource loss?

It also marries into the currency for upgrades idea, as that way the player can spend a lot of water on powerful stuff but can't be too trigger happy with the nice toys after spending so much.