The world of the Crimson Sun is a harsh, dry wasteland. Survival is hard, and thirst is the greatest danger. Caravans, armies, and adventurers must plot their courses according to the oases of the desert; and pay dearly to use the fortified oases held by the Merchant Houses along the trade routes. More intrepid individuals may try different routes, following maps bought at taverns, and hoping that the map isn’t a fake and the next oasis hasn’t dried out.
Finding Water: Anyone with the Adventuring or Survival proficiency can search for water. This requires a proficiency throw of 18+ (with no travel that day) to find an oasis or other large water source (which will not run out) or a proficiency throw of 14+ (can travel normally) to find small quantities (1 gallon) in cacti, plants, small animals, or little pools or pockets. A character cannot both hunt or forage for food and search for water. Characters with the Survival proficiency gain a +4 bonus on their proficiency throws to find water. The Judge can rule that no throw is required (in the rare verdant places where water flows freely), or that no water is to be found in the terrain (such as in obsidian plains). The Judge can also rule that a failed throw to find a water source indicates none exists in the hex.
The mounts and pack animals of the world of the Crimson Sun (giant ant-beetle-thingies, lizards, and hardy walking birds - which just use the stats for donkeys, mules, and horses) are all native to the dry wastes, and adept at surviving in them. They require 1 gallon of water per day (for a total of 1 stone/day in feed and water). Normal mounts (e.g. horses) would require 4 gallon per day of water.
Notes
There’s room for more detail (a list of modifiers or target numbers by terrain type, for instance; and possibly rules for wearing metal armor), but I just wanted simple rules.
The idea is to make water a real concern, and to make wilderness adventures dependent on access to water. Generally, a party will plan its journeys from oasis to oasis, usually among caravan trade routes, but even there, access to water may be compromised. Treks to dungeons, etc., may require obtaining maps (of often dubious authenticity!) showing oases on the way. Journeys into unexplored wilderness will rely on being able to find oases - but if the PCs can find a chain of oases at 1- or even 2-day intervals, they could create a new trade route between cities, for instance - one that might allow them to undercut competition with shorter travel times.
I think these rules and their application should create a lot of emergent gameplay. Getting lost in the wilderness could be a great danger (“We were supposed to reach that oasis yesterday!”), and an Explorer with Navigation and Survival is a huge asset.
Unlike in certain other games, create water is a 4th-level spell, so low-level parties will be particularly concerned with water. And even at 6th level and above, a cleric is going to be exchanging dispel magic or cure serious wounds or neutralize poison for that water. I’m pretty comfortable with letting 6th-level and above clerics support small parties (and, at 9th level, even large ones!) with water. A 14th-level cleric could lead 1,750 men through parched desert - an impressive feat, and huge advantage, but also a great expenditure of magic.