How does cold water help to overcome high fever?
Fever is triggered when pyrogens cause release of prostaglandin that acts on hypothalamus (which is the center of thermoregulation).

Fever assists in the healing process by:


enhancing phagocytosis
decreasing negative effects of endotoxins
stimulates proliferation of lymphocytes


Although is a good reaction, when the temperature gets too high it could determine negative effects, like convulsions at children. Brain damage appears when the temperature rises as much as 42 degrees Celsius, and most febrile conditions go up to 40.6 degrees Celsius.

So fever must be kept under control, and cold water helps lowering body temperature. Lowering the temperature will prevent negative effects of fever. Cold water is a symptomatic treatment for fever. Most important is to determine cause of fever and do an ethiological treatment.

Source: Wikipedia contributors, "Fever," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Fever&amp;oldid=614212619 (accessed June 27, 2014).