Unrelated, but interesting in this context: Michael Burry, one of the guys who predicted 2007 housing bubble (and made good money thanks to that), is investing in water. Looks like he might be right again.
In 2007, the reading list for my Political Economy class had a book focused entirely on the water wars of the future. Lack of clean water isn't a hidden opportunity, it's a planet-altering challenge.
1. Buy-side - ETF for companies leading in clean water or purification, or the second level that companies provide components for those companies in the first group.
2. Sell-side - bet on the companies will do badly in the situation e.g big polluters.
But the hard thing is timing, you can sit there for decades for the outcome and so many factors can affect your bet between now and then.