its molecular compisition.

The question should not be why is water transparent, but why aren't some other liquids/solutions? Water is transparent simply because it does not absorb light in the visible spectrum, and all frequencies of coloured light are transmitted through. Other solutions abosrb light in the visible range, for example a solution that aborbs red and green light will appear blue, as that's what left. Light is absorbed as certain frequencies of light have the correct frequency (and therefore energy) to be absorbed by electrons in a molecule, and the electron is then excited. Obviously, this frequency (and so colour) is then absent from the transmitted spectrum. It's quite complicated chemistry, and has a lot to do with molecular orbitals and quantum mechanics, but I hope I've underlined the basics.

Because water absorbs very little energy at the frequency of visible light. The graph below shows the way in which water absorbs energy of different wavelengths, and you can see a bug dip with the visible spectrum on the right hand side of it. This graph is taken from http://www.lsbu.ac.uk/water/vibrat.html which discusses the matter in lengthy, and rather technical, detail.

Pure water is transparent to the visible wavelengths of light, with maximum transmission of light in the blue portion of the visible spectrum. It also transmits reasonably well many of the wavelengths used by plants for ____________, thereby supporting plant growth in oceans and other water bodies. Water vapour in the atmosphere, unlike liquid water, absorbs the sun's ultraviolet radiation which otherwise would be extremely harmful to life on earth. Both liquid water and water vapour are nontransparent to the infrared radiation which is given off by the earth, thereby preventing much of this heat from being lost to space and keeping earth's temperatures at habitable levels. Water is a transparent, odorless, tasteless liquid composed of the elements hydrogen and oxygen. Water is an universal solvent, meaning that many elements can be dissolved by water. Fresh water has relatively few elements dissolved in the water, while the oceans contain many dissolved salts. Water is important to our lives, and without it we could not live. In fact, there are no living creatures that can live without water. Water most probably originated on this planet as gases were emitted from volcanoes. The Earth's atmosphere captured this water and has continuously recycled it throughout time, in what is called the water cycle. Water evaporates and forms clouds – the clouds provide rain and snow – which is collected in rivers, lakes, underground reservoirs, and oceans – that are the source for further evaporation. Water is the perfect substance for the water cycle, because it has a high boiling point and a low freezing point.

Good question...some GREAT ANSWERS! It also made me think...unfortunately...not ALL water (that should be) IS Transparent... Good old pollution and toxic waste...

Would I be completely lost to say that water is not completely transparent? For if it were, the sea floor would be as bright as the day light above. The depth of water eventually defuses the light that penetrates it, and isn't that the most prominent property of transparency? I may be delirious.