Water is a problem in many places. I do grant writing for nonprofit and public agencies and have worked on a bunch of water well projects (and have in fact been working on a water project today). They're much more interesting and technically sophisticated than you might imagine, and water tables have been dropping in much of the country. Wells are also easily fouled. Fracking has been a problem for water wells in many parts of the country.

USGS has much of the better and more useful data about water quality: https://water.usgs.gov/nawqa/studies/domestic_wells.

Right now, wells cost a lot of money to drill—often more than $10K—and are easy to screw up. That may be part of the reason YC is looking for water-related startups.