You could simplify things by having a random events table(s) that players roll on after each game. Then have rules for how that affects the campaign.
This could be more interesting.
At the kind of monetary scales Inquisitors/Rogue Traders/etc operate at, their income would easily stretch to supporting their water/ammo/etc requirements (unless someone's been blatting off heavy bolters full auto or something). Pretend to be an off-planetary noble who's there to negotiate some kind of trade deal ("Yes. Yes, I, I'm looking - to buy some... sand.") and no-one will much question the fact you've got a lot of money, particularly if your bodyguards are big and thuggish enough.
At this point, the problem is really more about finding the stuff you need to buy. And failures would be important to the plot. If one warband fails to find enough water, then perhaps there could be a scenario about their attempts to get more - maybe they're trying to steal it from another warband, maybe another warband is deliberately impeding their attempts to try and force them out of the running through dehydration.
As a question, how will you deal with characters who have their own warp capable craft? They carry months (if not years) of supplies and will have their own on-board armouries, be it for simply for arming the thugs who keep the crew in line/defend against boarding actions, or dedicated personal affairs.
If push came to shove, few of my characters would have objections to shuttling down a couple of giant water drums and a big crate of ammo if they hit a real shortage of supplies...