I'm reading The Boer War by Thomas Packenham, and in nearly every battle, he describes how the soldiers spent so many hours on the firing line without water, and that this frequently led to a loss of will to fight, particularly on the British side. Why on earth wasn't it standard practice for soldiers during the conflict to carry some sort of canteen and rations with them into battle?
They did - the 1888 Slade-Wallace pattern equipment (which was terrible in every way, then replaced by the 1903 pattern, which also was terrible in every way, and then by the 1908 pattern which was absolutely brilliant in every way) included a wooden water bottle with a pewter and wood stopper.
It was carried by a leather strap slung over the shoulder, the strap being blanco'd white in most infantry regiments, or black for The Rifles.
Example here
However, if you've ever done any vigorous exercise in the South African climate for an extended period of time, you'll know that one canteen of water isn't going to last you very long at all!