Hi
I have had good success carrying water in 2 litre pop bottles. Cheap, recyclable, allows you to spread the weight around between your panniers and rack.They are also much more durable then the regular plastic water bottles. If something catastrophic happens, odds are your not going to loose all your water in one fell swoop. Only real down size is they take up space even when empty. I carried up to 12 litres ( 6 bottles, approx. 2.5 US Gallons) this way on a recent tour in Australia. Filter would have been lighter, but I didn't have one. Hope this helps
Cheers