Fourth line from the end in my op. 
So, if a ray hits a completely opaque face, it will continue to trace it further set by the bias value, to search for another surfaces because it presumes there might be a contact surface? This is whats really bugging me, even if it does, will it make any difference at all? And strictly talking about situations, where the liquid is in a completely opaque container with no openings sealed with dielectric materials.
Best regards,
A.
Oh sorry, I did miss it.
Yes, I see what you mean, in fact my geometry intersects the bottom too, making sure the only actually visible surface is the plane we started with.
What it really is for me is a safety measure.
It’s a lot easier for me to visualise how far the fog tracing will go at most (despite density), and it’s of course heritage of working with dielectric contact surfaces.
In any case, the effort is really negligible (pretty much any single-sheet object can be given thickness with one shell modifier, or for the precision jobs, an edit poly), and in cases where it produces no difference (like this one!) one still has a “correct” result, rather than a potentially wrong one (the client really wants glass bricks for that swimming pool…).
I traced the issue down to the fog material setup alone, please confirm this, in case.
I totally get your point. Put it in real, get it out real. And this goes pretty well for everything in renderers nowdays. 
I was just very curious about your opinion on this.
Best regards,
A.
I had a look at the scene too and, as others have stated, the problem seems related to having a sun shadow bias too high (0.2m in the scene).
For some reason the faces that aren’t affected by the water shadow despite being under the water surface, due the shadow bias, appear dark blue.
I found that if you enable ‘force back face culling’ in the general options the problem seems to go away, so it makes me think that there is a problem in the way shadows are calculated in this particular case, but I’m not really sure, maybe Vlado could explain why VRay behaves like this.
Why was the shadow bias set to 0,2m in the first place? Obviously, a 20cm displace for the shadow *will* cause visible changes when rendering a pool in real world scale. 
Best regards,
A.
Yup, simply setting the shadow bias to something more suitable for the scene will solve this problem as well as some other artifacts that are visible on the sample image (the strange shadow on the bottom right corner of the column).
BTW, the scene has an Architectural material on it, to be safe better stick to VRay materials.
i have never had problems using a plane for the surface of water. its nice and quick to setup and works
If you used fog for the water, and the shadow bias is too high, then V-Ray will figure out that the faces are inside a fog, but the shadow rays will start *above* the water and will never actually cross the water surface - so the fog will never be turned off and it will cause all shadow rays to end up blue.
The solution would be to reduce the shadow bias to something that makes more sense, e.g. several millimeters or something.
Best regards,
Vlado
cool
Awesome yea must have missed that i work so many projects in different scales i lose track sometimes. Thanks so much for the help guys u where very helpful !!!
=)