Water molecule icon is included into Medical Icon Set. You can view all icons here:[link]
These medical icons are delivered in sizes 16x16, 24x24, 32x32, 48x48, 128x128 and also 256x256 used for Windows Vista. The icons come in two color variants: 256 colors and True Color with semi-transparency. They also have several file formats, such as ICO, PNG, GIF and BMP.
Download link: [link]