For the analysis of liquids chemical, chromatographic, optical methods are used. Chemical and chromatographic methods allow to determine the concentration of components in liquids with high precision, but does not provide a rapid analysis of liquids and cannot be realized in the contactless mode. In a number of practical tasks of monitoring and diagnosing technical wastewater quality control of food production, environmental monitoring of natural waters are highly relevant contactless methods of diagnosis of liquid media with their possible implementation in real time. That such properties, optical methods, in particular, Raman spectroscopy ( RS) of light and fluorescence spectroscopy .

As you may know various spectroscopy methods will work for you but at first you should  determine which materials are in water and choose laser wavelength range with minimum absorption in water and good absorption in other materials. 

Good luck

Dear Layth! Your problem solvent  with laser application depends  from  water admixtures. In the case of  solid admixture (suspension)  You can use green or red CW diode  laser and registrate  the level of  light scattering by particles (Mie scattering). In the case of molecular admixture of unknown nature in water, to my opinion, it is better to make use frequancy tunable in near UV (350-450nm) laser and to  record  the absorption spectrum of  your water sample with posteriour comparing  results with absorpion spectra of  standard purefied water.

Since the water is clear in the blue-green region of the spectrum, the choice of laser wavelength are known.

In the study contaminants at the molecular level by laser fluorescence used lasers blue or near-UV spectral range, for example, He-Cd (441 nm) and other gas lasers, metal vapor lasers, semiconductor diodes. Of particular interest are tunable in the spectral range lasers, such as dye lasers (stilbene, coumarin). For the same purposes use also compact Raman fiber-optical spectrometers (YAG: Nd, 532 nm). With less sensitive than fluorescence, Raman scattering is characterized by high selectivity and is a classic method for studying the chemical composition, including liquids.

When monitoring the mechanical impurities in the water (size ~ λ) is most effective to use the method of backscattering of light (532 nm).

From the above it follows that the control of the purity of water (on the molecular level, and the level of contaminants) should use a combination of methods.

Best regards, Leonid