When cycling around Australia, my water carrying methods were:
1) Camelback: I got one with 3L bag and then fitted a second 3L bag so I could carry 6L on my back.
2) Water Bottle cages on the bike: 3 oversized bottles for total of 2.5L
3) Two extra 1.5 quart water bottles to stash in panniers for total of 2.5L
4) "Sea to Summit" water bag for 4L
5) Extra bottles of liquid brought along the way and stashed in pannier
When in the Outback, I'd typically carry 6-9L through combination of the first three options. However, the maximum amount I left with was from Roebuck Roadhouse with 16L of water on board through combination of all five options above.
I like the sea to summit and dromedary bags as they fold down small, can be stashed in a (cooler) pannier. When cycling across Russia my riding partner had a large 10L bag that we could fill up only at end of the day to carry enough water for bathing and cooking that last ~10km into camp. It would get lashed across a back rack.