This would be the best choice, perhaps, but there is a big difference in that you will actually have to work or at least exercise continuous control during the 20 year period, which is not the same as putting your savings into an asset class and forgetting about it. Besides even if you do own a business, it is not necessarily a good idea to reinvest everything back into it.
That would probably be a very competitive business, there is nothing difficult in processing water. So I doubt that these companies will be a good investment.
The "industry" would Exxon-ized into an oligarchy, dominated by a handful of firms that handle all aspects of production. I believe the buzzword is "horizontal/vertical integration".
I don't think governments will allow water monopoly... Electricity generation for example is state owned in most countries.
What's your point? Electricity in the US is deregulated in many states and is undergoing deregulation in the rest. It is pretty much an oligarchy. If you think water deregulation will be any different, you are very mistaken.
I didn't vote for commodities but if I had, I would buy the PowerShares Deutsche Bank Commodity Index Traking Fund ETF [DBC] - it is the most liquid and is well balanced. The other alternative for a diversified commodities ETF is iShares S&P GSCI Commodity-Indexed Trust [GSG], it is 67% invested in energy (oil and gas). These ETFs buy and roll the futures contracts for the commodities and collateralize the positions with 3-month Treasury bills.
But I am also interested why so many people voted commodities and how would they implement such strategy