When I had my CJ-7 I had a dual swing out carrier in the rear that held the spare tire on one side and three 5-gal. jerry cans on the other. I would typically carry 1 fuel + 2 water cans, but realized that 95% of the time the water in the second can would never be used. Similarly, when I carried 2 fuel cans and 1 water can, there was only one occasion when I needed to use the fuel in the second jerry can which I could have easily avoided by not passing up an opportunity to refuel. Eventually I came to the conclusion that being able to carry 15 gallons in fuel and/or water afforded peace of mind that I would not run out, but the reality was that I only needed one fuel can and one water can unless I was on a long distance trip in an undeveloped area.
A bit hard to see, but shown here:
Fast forward to now. My needs have changed and I now offroad in a Nissan Xterra. Interior space is too valuable to be taken up with fuel and water containers, and standard jerry cans (Blitz, NATO, Scepter, etc.) are tough to mount outside without a custom bumper and carrier. Jerry cans are also too wide for 2 to be laid flat side to side on my roof rack, and I don't want to deal with height/clearance issues presented by carrying them on the roof rack upright or on edge.
My solution has been to use Rotopax fuel and water containers, securely mounted to the roof rack. [2 ea. 2-gal. fuel + 2 ea. 2-gal. water.] I carry an additional Rotopax 2-gal. water container in the rear of my truck rigged with a siphon faucet for the convenience of running water for cooking, wash up, etc. I have never run out of water with this set up (6 gal. total), nor have I run out of fuel with 4 gals. available in the Rotopax containers. My Rotopax containers have never leaked, and the 2 ga. size is easy to handle. One 2-gal. gas can is about 13 lb.s filled; one water can is a bit over 17 lbs. filled; total weight for the 4 on the roof rack is about 60 lbs.
My current arrangement: