[vitohuang]

There are two sides

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.