Sure, there are so many great emerging water technologies - drip irrigation and conservation tillage, hippodrums making it easier to retrieve water, recharging aquifers with excess rain, grey water systems, saltwater adapted crops, life straws, desalinization plants, solar and ceramic water purifiers, and many more. Yet my choice is the humble rain barrel?
Where is the innovation in that?
Awareness. How would your water use change, if you could see your supply rise and fall with the rain.
Simplicity. If you have a roof, you can use it for rainwater capture.
Evening out surges. With increased climate variability, having a buffer capacity to absorb excess water would potential reduce damage due to flooding and filling up the barrel or cistern for use in the dry times is, of course, also a benefit.
So I nominate the humble barrel as the technology that everyone should consider.