Money is an uncountable noun. The best thing you can do is count units of money. And every nation has a different name for those units and they are valued relatively to a particular unit’s buying power at home and it’s buying power after conversion somewhere else. And this value is always changing. In other words, we all have a pretty good idea (at least we think we have a pretty good idea) of what money does, but the fact remains that money is completely relative to a extraordinarily complex set of factors, among those including political and economic ones. In a simple sense, diamonds are only valuable because they are rare, and gold (which used to be used as money) was like that as well. But now hardly anyone knows what exactly is determining the value of money- even the economists don’t know.
So how then is money like water? Because most of us have a very simplistic and narrow view of money, and in general, the more you have, the better. But money, like water, always has to go somewhere, doesn’t it? You can, I suppose, accumulate it and keep it, like a water reservoir, in a big bank account for future use. But water evaporates into thin air and so does money (through inflation or economic voodoo spells), so you either must then invest it and make money off your money (which is always risky in one way or another) or spend it ahead of inflation. In any case, if you’re not in the business of making more and more money, (if you’re on a fixed income) money can become a very difficult paradigm to figure out.
I don’t have to worry about that at the moment, though I do find that position enviable (well, a more or less fixed income- I’d like to deal in goods rather than work for cash). But right now my bankbook reads like a riverbed in an extreme environment of a monthly flash floods, immediately followed by a month long drought. Money flows in, I pay all the bills, and there’s nothing left to save. So the nation of me is very water poor.