This isn't a question of whether water will dehydrate; this is a question of whether businesses should be allowed to market their products as something that can prevent dehydration.
1) Water isn't the only product that will dehydrate you. Coca Cola will too, but if Coca Cola tried to say regular consumption of significant amounts of Coca Cola can reduce the risk of development of dehydration, we would rightly tell them to shove off, even though the statement is factually accurate.
2) Water will not necessarily rehydrate you, if you are suffering from dehydration. Many people have already said why: it's not just a matter of liquid, but also minerals, salts, and the presence of disease.
3) Water will obviously make you less thirsty. If you feel dehydrated - that is, if you feel thirsty - then it's pretty god damn obvious that drinking water will make you less thirsty. It is completely unnecessary to advertise the product as "drinking regular amounts of this product will make you less thirsty". See the cereal comic.
4) By advertising that this bottle of water can prevent dehydration, it implies that there is something special about this bottle of water. As I said, everyone old enough to talk knows that drinking water will make you less thirsty; if a product says that it is explicitly rehydrating, then that implies that there is something more to this water than simply being water. Perhaps it has special rehydration properties that I'm not aware of. Who knows.
For all those reasons, water shouldn't be allowed to be marketed as "rehydrating", or whatever. At best, it's a completely unnecessary statement of the obvious, and at worst it is a dangerously misleading ploy to get you to buy a product that comes out of your taps for 0.1% of the cost. Laws and regulations exist to benefit society at large. What benefit to society is there of allowing marketers to advertise water as rehydrating?