I'm sorry, but I'm confused about what is probably a simple thing... A Magus is suffering from thirst while crossing the desert. Can they conjure water to quench their thirst?
Ch9 p324 – Creo Aquam guidelines
Water created temporarily quenches thirst but provides no lasting benefits to the drinker. It can be used to wash, and the things washed stay clean.
Sounds like no, but...
Ch7 p214 – The Limit of Creation
Hermetic magic is incapable of creating anything permanently without raw vis. This limit affects all uses of Creo magic. However, as pure Creo magic does not violate the essential nature of its target, Creo magic that does use raw vis does not need to be maintained.
So, yes, but it requires vis? (And if so, where is this explained?)
If so, what is the vis to water ratio? Would it be easier to carry water than to carry enough vis?
A magus who non-ritually co jures water to quench their thirst will only have it quenched for the duration of the spell- so a mage who drinks water conjured everyday from a Moon duration spell will probably be fine, as by the time the water dissapates it is ejected as waste and replaced with other magical water. There is the issue however of the spell ending prematurely, as might be the case entering the Magic Realm or some regios.
That said- It doesn't really answer your question. It would be easier to carry water, as magi generally see vis as super valuable and you'd need to conjure water every 3 days or so. Canonically, magi often also rely on mundane logistics.
Non-ritual water doesn't quench thirst-if the magus has the spell expire before they can probably hydrate themselves, they'd suddenly have to do some deprivation checks.
When you cast a formulaic or spontaneous spell, the water will only temporarily quench your thirst, but if you cast it at Moon duration, you should be fine, unless you've only relied on conjured water for most of that time.
If you research a ritual spell, and cast it, using 1 pawn of Vis per 5 levels of the spell, the water created will be permanent, and you can drink them will no ill effect,
The amount of water created depends on the spell. The higher the spell level, with more Size magnitudes, you'll create more water.
Taken out of context those two quotes can indeed be confusing. Here's how it works. In ArM5 anything created with a non-Ritual Creo spell is magical and ceases to exist as soon as the spell expires. However, in the description of D: Momentary, we read:
Ritual Creo spells with Momentary duration create things that last as any other thing of that type. The magic is gone in a moment and so cannot be dispelled. This also applies to ritual healing spells
Thus, it is possible to create "natural" water (and/or food), non-magical and nourishing, by employing a D:Mom Ritual spell, which costs the usual 1 pawn / magnitude.
In previous editions it was often possible to "boost" the duration of magical spells with vis (1 pawn/magnitude); and indeed it was often the case that spells cast without vis would create something lasting a day or a month, but with the addition of vis they'd create something permanent (either permanently magical, or permanently"natural" as per D:Mom Rituals in ArM5). The Limit of Creation is a "legacy" law of Hermetic magic from back then, stipulating that whatever magic one used, only by adding vis would one be able to get such lasting creations, including permanent healing and nourishing food. Here and there traces of the old wording remain, and this can be a source of confusion!
We actually had to do this on a recent adventure when we found ourselves at sea for an unexpectedly long time. My mage created Moon-duration water and food with spontaneous spells, and made very, very sure afterwards that we all had plenty of mundane food and drink before the next full moon. And no, he didn't care if you didn't feel thirsty, drink anyway.
Chapter 9, p.308 (Definitive preview PDF), p.112 (5e core), under the Momentary duration:
Ritual Creo spells with Momentary Duration create things that last as any other thing of that type. The magic is gone in a moment and so cannot be dispelled. This also applies to Ritual healing spells.
Chapter 9, p.315 (Definitive preview PDF), under Ritual Spells
If the spell is a Momentary Creo spell creating a lasting thing , it must be a Ritual.
(the equivalent heading on p.114 of 5e core does not have that bullet point.)
The "ratio" is accordingly however many pawns of vis it takes to cast the spell (that is, the Base Individual of Aquam involved and what the Target and size of the spell are).
Note that despite some statements from other posters, exactly what happens if you use (say) Moon duration magic for food and/or water is not unambiguous; there are semi-contradictory statements in the rules, and different Storyguides prefer different reconciliations. A forum poll in 2006 on magically-created food demonstrates the wide spread of opinions, none (at least then) commanding a majority. Definitive uses the same text as quoted in the first post of that thread.
I'm inclined to have Moon duration food and water from standard Hermetic magic sustain life, but with something like deficiency diseases. And warping points.
Here we are, visiting the covenant of Carrenae, where they've lived on conjured food for decades. They're all pale and really thin, with oddly long ears. They're clearly repulsed by the wine and bread we offered as a courtesy, but won't stop commenting on how "plump" we look.