A friend of mine is a retired Surveillance Photographer for Special Branch in the UK. He used an amazing assortment of kit for his work from sub-miniature Minox cameras which fitted into a cigarette packet and allowed for a number of real cigs too (the lens was hidden in the pack logo!) to 1200mm lenses on 35mm camera bodies (was Nikon at the time
) when working from a hide (often a van or truck left parked some hours earlier with my friend left inside for many hours with a tripod, sandwiches and two bottles; one with water, the other empty.
His advice would be NEVER mix up the bottles!