Me, I've got a 2L Camelbak plus a 2L and a 10L Ortlieb water bag. The advantage of these waterbags is that they need very little space when empty and can be hung up when in use when at camp.
In Oz I met a guy with a 650GS Dakar - he had removed the right muffler and fitted a watertank of the same size. For what reason does a single need two mufflers anyway ;-) Additionally he had fittet two tubes on the frame behind the front wheel - one for tools and the bottom, bigger tube as water tank. I thought that was rather cute.
Only when we pause to wonder
do we go beyond the limits of our little lives.