I have an Ortlieb 10 litre waterbag which I can recommend. I have used it for many years and have dragged it all over Africa in a truck, through North Africa and Europe bungeed to the back of my bicycle and in my rucksack whilst 'wadi-bashing' on digs I worked on in the mountains of southern Jordan. It has been really robust, been dropped, dragged and never leaked once; and it has the added bonus of having a shower rose adaptor which you can screw on, hang it in a tree and get yourself clean at the end of a day.