Water filtration systems are great, but they require a lot of money, and they'll take a lot of time. There are steps short of distributing water filters to everyone or building water treatment plants everywhere which we can start with-- the money and the time for these other things will come later.
I decided that the favorite water security program I saw was the World Bank Scaling Up Sanitation
Project. They estimate that 2.5 billion people do not have access to basic sanitation services. They are trying to educate communities about Open Defecation, and convince them to build latrines. They call it "Community Led Total Sanitation and Sanitation Marketing". Develop demand for latrines, and social pressures against pooing in random places... that's the focus. They've reached nearly 4.5 million people.