I am pretty much against those water bags, we aren't camels ... I prefer plastic bottles. You can buy 750 cl size and have one on each side of your backpack.
The main reasons are that using a tube to drink as you go - apart from being a little too Star Wars for me - is that you don't stop walking! Also, you don't know when you are going to run out of water, then there is all the opening of the pack and removing it to refill it, then the cleaning of the nozzle and so on -
With a bottle you can find a nice spot, heave off the pack, take off the boots and socks and let your feet breathe .. then whilst you look back to where you've been take a big swig and look around, smile and wave at passing pilgrims ... so much better than this para-military 'don't stop, keep going' mentality.
Also - you can share a bottle .. lots of other pilgrims on the Camino you know.
You can wash fruit with a bottle, pour water over your head or down your back with a bottle ... pour some out for a hot dog or cat ..... water a Camino-side plant that looks dried out and gasping ...
You can balance the weight by drinking alternate sides - wrap them in baco-foil to keep them cold if you want. Easy and fun to fill at fountains - an excuse to stop, to enjoy, to be there
so, for me, bottles every time!!! :wink: