Water is heavy and carrying it on your back with the rest of the gear can be quite uncomfortable. I'm currently using a PLCE belt webbing set with 2 double PLCE ammunition front pouches and hippo pad which gives space for 3 Lifeventure 650ml Tritan Flask bottles (Nalgene compatible) and a Katadyn Ceramic Mini Filter Pump with a Nalgene Active Charcoal adapter. Having 6-7 lbs on the front/side of the body helps with the load distribution (i.e. not having an additional 6-7 lbs on the back and shoulders) and it makes filtering and refilling the flasks quick and simple as well without having to remove the back pack and put it back on!

http://www.lifeventure.co.uk/htm/cookware/tritan-flasks.html

http://www.strikeforcesupplies.co.uk/index.php?method=stock&id=9012&from=48