It makes perfect sense to me. It's HOT in most parts of Australia; water is essential for survival. Even if the water systems have long been de-pressurized and out of service, there might be just a few cups of water inside every hydrant, ~IF~ you can open them. You might need to insert a rubber hose and suck the water out, but there would inevitably be some water down there. A man who carries a hydrant wrench would have a huge survival advantage. The average city has hundreds of fire hydrants. Even after a nuclear war, the hydrants and underground pipes would still be in place- even if the whole city is gone.
I'm not saying it's a useless tool, and you illustrated some possible reasons why it may make good sense to have one. But why would you carry this on your person at all times instead of leaving it in your vehicle (like the gas cans, food, etc) in favor of something more practical that you may need at a moment's notice? It also may be too literal to think of using it for it's intended purpose, but the hydrant wrench doesn't look like something that would be a really good club or pry bar. My best guess is that it's a common enough large hex size to be useful, and has a wider span than the crescent wrench can adjust to.
Yeah in a post apocalyptic world, clean water is probably more precious then gasoline. You can last just days without water, especially in a desert.
Totally agree and Fury Road definitely supports that. That's why it is odd that Max carries a hydrant wrench with him and not a canteen. If you keep all your water in your vehicle and get separated from it in the wasteland, you're probably screwed. The only thing I can think of is that there must be a greater risk for carrying water and having people try to kill you for it than the risk of being separated from it.