Folks,
After having received your feedback we went back into the pool again and tried to go for pictures where "the underwater" element is more obvious.
This time I used a Nikon 17-70mm lens (instead of the 12-24mm) from Tokina. Using the same Ikelite 8" doom. I got the impression that I suffered more from CA with the Nikon lens at 18mm then with the Tokina at 15mm.
We used buddy lines and ribbons to strengthen the "underwater idea". The black & white (picture 2) is my favorite. Evelyne, the model likes the picture 4. I do not like it all that much…
Picture 1:

Picture 2

Picture 3

Picture 4

Cheers,