How best can you invest in water treatment, water rights, water management, etc to profit in the future?
Oh my goodness, you've asked a question that I am SO going to overanswer.
I work on this problem on behalf of the federal government, so I'm not allowed to invest in the relevant places. With that said, I have seen a few different groups of people make large bank by playing the water market. Some local examples...
1. The Columbia River is dammed for hydropower generation by the Bonneville Power Administration. They have a forecasting model for how much water is going to be available, based on the altitude distribution of this year's snowpack in Canada and northern Washington. Water is bought and sold with futures contracts, just like pork or oranges. The BPA model isn't very good, compared to the state of the art stuff, so there's a whole secondary market of people doing alternative water forecasting models and then selling that information to private traders. In years of bad snowpack, like this one, the price of water goes up.
2. Some state's have laws that are more protective of private water rights than others. California is basically a free for all wild west race to the bottom of the tragedy of the commons type of scenario. In Washington, we have water rights allocated based on seniority, with junior users having their water shut off in drought years before more senior users. The state is just now moving towards considering groundwater and surface water as a single combined resource, so some people are speculating that junior groundwater uses, which are currently unregulated, will soon be curtailed in the same way as junior surface water users. That would create a market demand as those junior gw users look for alternatives.
3. In fully allocated basins with no more legally available water rights, there are always private water rights available for sale. One developer in Washington spent a large chunk of money in the mid 2000s to buy up local water rights from people who weren't using their full allocation, with the intent of using that water to build a resort. Then the real estate market tanked and the resort wasn't financially viable anymore, but concurrently the price of water in their basin went through the roof and they've made back all of their money and more by selling off little pieces of the water they previously acquired at rock bottom prices.
4. Agricultural products have different requirements for soil type, climate, and water. Some crops are much more water efficient than others. Farmers in these parts are busily tearing out vast swaths of tree fruit orchards (~38 inches of water per year) to instead plant wine grapes (6-10 inches of water per year) because they make more money on the product for less money spent on water. This trend is why Washington is now the number two wine producing state in the nation. Future changes in regional climate are likely to open up more land as suitable for wine grapes (by raising temperatures) at the same time as water supplies continue to diminish, making alternative crops less profitable. I'd be comfortable betting on Washington wineries going forward, less so betting on pears or cherries or apples. Though it's always possible the farmers will overshoot and undersupply the demand for those fruits, pushing the prices back up.
5. Technically we have plenty of water on earth for everyone. The hard part is getting it clean and delivered to where it needs to be, which is why vast sums of money are being spent on water infrastructure. Desalination plants in California are a good example. Here in WA, there's a $5 billion plan awaiting Congressional funding to significantly alter the way a single basin operates their reservoir and canal systems. If it gets funded, that work will employ a significant fraction of all construction firms in the region for roughly a decade, to complete a project that would provide a reliable and safe water supply to the state's agricultural interests.
6. Some industries are more water intensive than others. All of the big internet companies now have server farms along the Columbia River, because it provides abundant water for cooling while simultaneously providing cheap (renewable/green) hydropower for power-hungry data crunching. Semiconductor manufacture eats water. Cotton is a killer. Wood and paper products are harsh. These industries are going to have to relocate to places with abundant water, which means you could bet against them in places with coming droughts. In WA we have an exemption for groundwater wells for Confined Animal Feeding Operations, so big parts of the state are packed full of dairy lots because even though dairy farming is super water intensive, there's a legal loophole for it. For now.
7. Utility companies are cash cows. People need power and water in their homes, and they will pay whatever price the utility demands to provide those services. Some utilities are legally restricted to essentially operate at-cost, like a non profit, for the benefit of the customers. Others are privately owned profit machines that pay 5% dividends. Income from utilities is remarkably consistent over time regardless of price fluctuations in they underlying asset, because they have large fixed infrastructure costs. So if the price of power/water goes up or the demand goes down, they just adjust their pricing model to ensure the same revenue stream. Some utilities would make a good hedge in a resource portfolio. Others are in the business of mergers and acquisitions, buying up neighboring utility companies to capitalize on economies of scale.
Water is a local issue, right down to the basin scale. Like any other business, investing for profit means having a detailed understanding of local market conditions, the applicable regulations, the current players in the field and how they do or may interact, and then getting lucky with finding the right opportunity. There are already lots of smart people playing this game, so I wouldn't expect to cash in with a hot tip or some vague notion that water is getting more scarce over time.