The ‘Water’ material in the library is set up using white refraction and a ‘Fog Color’ which is giving the material it’s tint/color.
The ‘Fog Color’ will accumulate based on the volume size.
To make it more transparent, you can decrease the ‘Fog multiplier’ value.
The appropriate value will be scene specific but you can try with 0.01 or even smaller values like 0.001.
When setting up completely refractive/transparent materials with V-Ray the ‘Diffuse’ color has to be set to black.