Barry,
Thanks for letting us look at your file.
To get the transparency perfect you do need to use the rendering interface as mentioned earlier.
Our OpenGL display has been designed to favor responsiveness over perfect rendering accuracy.
As transparent surfaces stack in a given drawing the cost in CPU cycles increases. To get it right would be to sacrifice refresh speed in the 3d window. In large models this can already get slow. So it was determined that ArchiCAD's OpenGL 3d window will get one layer of transparency correct. Depending upon the order of processing distant objects it may get two or possibly more layers correct, but this is not guaranteed.
In your swimming pool example the processing order of the tranparent polygons in the picture, by luck, get it correct, on one side or the other.
That is, this is a bug that we will live with until processors are better.
Graphisoft Technical Support