Trauma, blood loss and thirst
After some general research there are specifically 3 arguments that I came across that people give for supporting this particular theory.

1) somebody seriously injured might be given an anesthesia shot by the doctor or whoever that treats him first up for relief from the pain and distress. This would lead to relaxation of the stomach muscles and the contents of your stomach can travel up to your throat and into your lungs which would be fatal. Read about pulmonary aspiration. 

2) the second case is if there is an injury to the gut or the GI tract, then providing water would be counter productive. A layman might not be able to assess the level of injury to someone just by sight. 

3) the third is that giving water would increase blood pressure and (sometimes) apparently dilute the scars. This could also lead to chances where sutured wounds might reopen.

There are no real guidelines as to how much water can be given to an injured person. Depends on the situation or how much time he has been bleeding for and the kind of environment that he is subjected to like you pointed out. Some medics have recommended sucking on citrus fruits to prevent dehydration. Also, swabbing the patient all over the body with a cold compress can provide some relief and delay dehydration.

Guidelines for bleeding victims are here.