Tubus racks are awesome, and you'll be very happy with them. In terms of carrying water, you have a few options.

Mount bottle cages everywhere you can. Depending on the frame size, most touring bikes can handle three or two (for short folks like me). That's enough water for an hour or two, but what then?

• You can pack some water bottles into a pannier or a rack pack. Harder to get to, but when you're touring you've already got the bags on. Front panniers are great for this, since you can get to them while straddling the bike.
• If you have the room on your seatpost, you can get a double bottle cage that mounts behind your saddle.
• Ortlieb panners will accept water bottle cages, but you have to do a little surgery on your panniers, and you can't use standard cages. Handy if you already have those bags, but they can be finicky.
• I keep a camping roll strapped to the top of my rear rack, and can usually find a place to bungee in some water bottles. If you're clever, you can get to these without getting off the bike.
• Camelbaks are great if you don't want racks or bags. I use a water bladder when backpacking, but never on a bike. Why get my back all sweaty when I have a bike to carry stuff for me?