Water
Water is the lifeblood to our survival... just above food. Without water, you die.
You need to know how and where are the best locations to capture it on your land/home/property, then you need to know how to store so that it doesn't become tainted, and you also need to know how purify it for drinking/cooking/laundry (as your situation prescribes).
However, when it comes to waterproofing your newly finished basement... water is a colossal pain in the ass!
That's all I have to say about that.