Hi Piet,
I'll have a look at the POV, but what you say about the altitude is my fear as well, hence my challenge to the wizzkids. On the other hand, if you have a surface shader there's the possibility to give the soil at a certain height a colour (see my red-stained underwater area), why can this (coloured area) not be translated to the water plane (actually the attached foam layer)? I tried all sorts of functions, another water shader, colour to vector, vector to colour, etc, etc. No luck so far.
Perhaps mixing to worlds together, one with the water, one without, where the altitude is based upon the waterless world and transferred to the waterworld might do it, I don;t know... just another wild guess.
---Dune