What is a water exchange-traded fund (ETF)?

Could you please explain what a water exchange-traded fund (ETF) is? I'm curious about how it works and what makes it unique compared to other types of ETFs. Is it specifically focused on investing in water-related assets or companies? What are the potential risks and benefits of investing in a water ETF? How does its performance typically correlate with broader market trends or changes in water availability and demand? Thank you for your insights.