Gamp's Five Laws of Elemental Transfiguration specifically state that nourishment cannot be conjured from thin air. Then how can the aguamenti spell work? Does it just take water from somewhere else, or is this a chink in Rowling's armor?
2 options here:
Food (and not "nourishment" as you phrased it) does not include water. Hermione says:
Food is the first of the five Principal Exceptions to Gamp’s Law of Elemental Transfigura—".
In common English, water is usually NOT referred to as "food".
Since water - unlike non-water food - is super-plentiful and super-fungible, clearly it's easy for it to just be transported from somewhere, anywhere else, even if we don't assume that "food" excludes water.
It actually specified that "Food" cannot be created out of thin air, it doesn't specify water. Additionally, the "air" itself contains quite a bit of water. Put a cold glass down during hot weather and you can "Aguamenti" some water for yourself.
As far as what it actually does; it conjures up water from the tip of the wand in varying amounts (seemingly depending on the user's intentions and concentration).
These answers have pegged it pretty much, but this is something to add that may apply - at least, I've found no evidence that it cannot apply: You can't drink it. The water produced by the Aguamenti charm may not actually be drinkable. Never, as far as I can see, does somebody drink water made by that spell; and in addition, in Half Blood Prince, Harry cannot get the conjured water to Dumbledore's lips - likely this is Voldemort's doing to force people to go for the lake water, but then, it might be because Aguamenti water vanishes if it is to be drank. A long shot, but a possibility.
edit: As @ibid pointed out, Wonderbook disagrees. For some reason JK's canon is higher than mine (I don't get it either) but there we are. Augmenti is fully drinkable!
That doesn't technically mean Voldemort didn't put a spell on the cave to make summoned water undrinkable (seems weird to me anyway that it is drinkable considering food, and I would assume drink, cannot be produced magically, but that's another question entirely 《and one I could probably answer if I wasn't taking the opposite point of view here!》) but that's clutching at straws here and isn't really even hinted at in canon so ...
Yah, nevermind! It's drinkable.
I think Aguamenti merely conjures a stream water , the same way "Avis" conjures a flock of birds. Now that water bay be real water summoned from a stream, or "magical water" that will eventually disapear just like leprechaun gold. But as the name of the spell suggests, and if Luna Lovegood is to be believed, it results from the action of a water counterpart to helioapths the "spirits of fire".
I don't think aguamenti is such a powerful spell. Even muggles can produce water from air, by cooling it down. It's not really creating anything, it's just extracting it.
I'll tell you how I interpret this rule about nourishment: I think it's a ban on making anything from air which it didn't already contain. For example, a plate of sandwiches may be summoned from the kitchen, but so must a bunch of flowers, for they are nourishment of a form. Or - let's say - a chair. I know many wood-worms who are quite insistent on this point. Such magic would be in the arena of potions and transfiguration, respectively.
(Although... if it's not banned to mention such hereticism in this page; Eliezer Yudkowsky in "Harry Potter and the Methods of Rationality" deals very thoroughly with laws relating to, and dangers of turning inanimate objects into food.)
In my estimation, if the water is summoned from some other place, it would summon only the H2O molecules, not any substances dissolved in the water. This would have the same net effect as distilling the water. It would be completely pure on a molecular level.
It's not advisable to drink more than a small amount of distilled water by itself as it can upset the balance of electrolytes in the body. It can still be used for cooking, of course, but beverage water should contain some degree of dissolved minerals as all natural sources of water do. This could be why wizards don't typically drink the water produced by the spell. Distilled (pure) water also has a taste that many people find unpleasant, due to the complete lack of dissolved substances.
At least, that's how I will approach the subject in any fanfics I write that discuss aguamenti.
If I may, I'm going to post a short explanation of magical exceptions for a fan fiction, which I based largely off of real physical limitations dealing with energy. The 1st Law of Thermodynamics, for instance, states that energy cannot be created or destroyed, only transformed. I feel that, with respect to Gamp's Law, this would still hold. Also remember that Space, which I define as cosmic energy, is a limitation of magic (see: Department of Mysteries, Hall of Space) which wizards do not fully understand.
Five Exceptions: 1) Sustenance - The nutrients and properties of plants, most usually labeled as 'food,' must be formed through the natural processes. One could create a food substitute, but it would not have any nutritional value and would not save you from starving. Nutrients are gathered from the Sun's cosmic energy and transmuted by plants to form nutrients, and as magic is limited by cosmic energy for reasons yet unknown, they cannot be formed.
2) Earth Metals - As with nutrients, earth itself cannot be created. Other forms of earth can be transmuted into a less-than-perfect imitation of another form of earth (nickel to gold, the Philosopher's Stone, for instance), but this can be easily detected by experts such as Goblins. Water is created by magically deriving it from the air around a wizard, fire is simply combustion, and air is again a series of compounds which could (with great difficulty) be replicated. Earth, however, is of raw material, and raw material cannot be transfigured since it is of cosmic energy.
3) Knowledge, and certainly not wisdom - Knowledge - including prophecy - cannot be transfigured from the lack of knowledge. It is a chemical process of memory, by experience, that cannot be replaced. You can transfer knowledge from one body to another, as with pensieve magic, but knowledge itselfcannot be created.
4) Love - Love, as always, is the most mysterious of states. Not like the chemical notion of infatuation, which can easily be replicated, love is a purer concept that is yet to be understood. As such, it cannot be created from thin air.
5) Soul - Life, perhaps more confusing than love, is the most assured exception. A soul cannot be created, nor resuscitated, by magic. The most powerful of necromantic practices ever established have proven to only accomplish the creation of wraiths - shades of souls which are made by deriving the energy left over from a being's existence, be it memories or influences or even imprints upon objects. Similar shades of life, such as the birds created from the Avis charm, are not real life but imitations of creatures that behave as they would. Note: the Philosopher's Stone does NOT create life, but extends it. Think about how wizards are able to transfigure foods to create more of it, such as stew in a pot that needs refilling. The same idea applies: the stone merely magically acts upon the physical body and maintains the creation of new cells, which is the vehicle of the soul. Since the body is still around, so the soul will be.
--- As an answer to the overall question, the Aguamenti spell is a charm rather than a transfiguration. It probably works either by summoning water directly or by combining hydrogen and oxygen(2) within the air around the wizard to form new water.
Sorry if this is too scienc-y for magic, but whatever.