Gelert 2LTR Hydration Pouch.
Dehydrated is the most important condition to avoid. It will sap your energy and in the extreme can be very dangerous. One way of avoiding it is to make sure you carry plenty of fluid and drink frequently. Dehydration doesn't show itself making you feel thirsty either so drink frequently even if you dont think you need it. A simple yet effective hydration pouch from Gelert is an ideal addition to the kit.
It has a capacity of 2 litres, enough for a few hours and is simple to fill and use. A small price to pay for such a useful piece of gear.