The issue, of course, is not "water"; it's freshwater. We have a lot of water on this planet. Generally it can exist in 5 states: seawater, clouds, freshwater (or what I like to call "drinkable land water"), aquifer water (underground water), and snow/ice.
Around the world aquifers are being depleted. This is a problem because this is one of the most low-energy (and technologically well understood) ways to harvest drinkable land water. And humans are not the only living creatures that use aquifer water! If there is not aquifer water for plants then the plants are completely dependent on rainwater or flowing drinkable land water (rivers, creeks, etc., which are all on their way to becoming seawater again ASAP). This is a precarious state to be in, because on a macro scale, once plants start to be incapable of doing their job (providing ground shade, ecosystems for biomass, improving and retaining soil structure, etc.) a landscape can be on the road to desertification. What does this mean? That means that it's going to stop raining. This has happened, many times, because of human modification of the landscape and has led to the total collapse of multiple powerful civilizations (Jared Diamond's book "Collapse" talks about things like this).
So what are we supposed to do? Say you are an ecological steward (or policy maker) for a couple hundred acres of land that are on their way to desertification or that are already in a stable, but arid, water cycle. It is easy to think of water in terms of accounting and cash-flow, what is the big picture that will make the landscape profitable and growing in "financial" reserves?
The big picture is very simple: we are trying to make seawater into permanent land water. The more net land water the Earth has, the more stable and abundant the existence of terrestrial life on this planet, in general, will be.
(Just remember we're practicing for Mars!)
How do you do this? The input of "free" water we have (meaning no energy cost for the conversion from seawater to potential land water) is rain. We need to make sure that as much rain as possible stays as underground water... or the *sixth* form of water that I haven't mentioned yet: biomass! There is a lot of water in biomass. And it is a relatively closed loop (meaning that once some water becomes biomass it will stay in the biomass cycle for a long time). Insects, plants and *especially* soil biology are some of the greatest resources we have for storing water on land instead of losing it to the ocean.
And then of course, we are all technologists, so I think it is also worthwhile suggesting that we should be using renewable energy resources to desalinate saltwater and just pump it back (I don't know if these techniques have even been invented yet) into our aquifers and ecologies.
