Make the manually poster with water drop falling in Photoshop.
Source link: http://www.photoshophelps.com/2016/03/28/poster-on-save-water-save-life-photoshop-tutorial/
If you want a banner on a water topic easily and fast. Just go to photoshop and do easy step with best tools like brushes etc.
If you have event in public place of water consumption. and you have need a water poster for wonderful event. Do your own with Photoshop using some wonderful tools. Make new file, apply artfully on the poster.