Here are some existing poster from around the world promoting "Water is/for life"
The first two aren't bad, the first one is a typical type of poster you'd get from a government agency, but nothing really stands out.
I like the concept behind the second one, using the water drip and putting images of "life" in it.
The last two are quite nice and smart. Using the image of the tap and showing if you save water you can save lives ie the fish.