My thoughts and advice:

I use a 4L MSR Dromedary. It's suitably sized for moto-ADVers (they do have larger and smaller sizes too), it's hard wearing, you can hang it up at camp and it's easy to clean. The 3in1 cap is a neat idea so filling, drinking and pouring is simplez - no spillage. I like the wide mouth as I can pack it with ice cubes in the summer to keep it cool. You can get hydration and shower attachments for it too if you want.

4L is usually enough water for me for 24 hours. I do carry a couple of empty 2L Nalgene Cantenes/Platypus' rolled up in my kit in case I want to carry extra. I try to avoid hard bottles as they take up room when they're empty. If the tap water is drinkable I just ask at gas stations, cafe's or shops etc to fill them up for me. I've never been refused yet. It tends to spark a conversation as most people are interested what you're doing. Don't waste money buying bottled water if the tap water is good and in most places it is.

I used to go with the don't put all your eggs in one basket approach so I started by carrying 2 or 3 different sized water bladders but it was a PITA when asking people to fill them up for me so now I just use the one 4L Dromedary. If I know I'm not going to be stopping all that much and its going to be hot day I'll put one of the spare 1 or 2L hydration bladders and hose in my tank bag and drink from that while on the move.

I also carry an MSR Hyperflow micro water filter, it's small, lightweight, very effective (down to 0.2microns - bacteria, protozoa & particulate is no problem), it's very easy to use and super quick (3L a minute). If you're unsure about the tap water just run it through the filter (here's where the spare bags come in useful - don't use your clean bag for suspect water). Of course you can use the filter in streams etc. I have some water sterilization tablets as a back up and you can always boil the water too if required.

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www.AdventureVagabond.com Mongolia & Siberia 2012: Adventure Moto Madness