Quoted from
http://www.felinecrf.org/nutritional...ry_v_wet_food:
As desert animals, healthy cats do not naturally drink a lot, instead
they obtain moisture from their prey (a mouse, for example, is around
65-70% water); and whilst cats who eat dry food do tend to drink more
than cats fed on wet food, they simply cannot take in as much moisture
overall as a cat fed on tinned food. This was known back in 1986, when
the US Board of Agriculture stated in an article entitled Nutrient
Requirements of Cats.

More recently, an article entitled The carnivore connection to
nutrition in cats (no abstract provided, but the document may be found
here, or here in pdf format) (2002) Zoran D Journal of the American
Veterinary Medical Association 221 pp1559-67 states:
*
"cats eating commercial dry foods will consume approximately half the
amount of water (in their diet and through drinking), compared with
cats eating canned foods... In older cats that tend to produce urine
with a lower concentration, an increase in water consumption becomes
even more important to avoid dehydration and development of prerenal
azotaemia".