I have the HeinrichsWeikamp converter in my Aquatica EOS 5D housing and it works great with my Inon Z240. I did have a problem at first, which was that when Aquatica originally sent me my housing, they didn't velcro the board in properly. It was dangling during shipping and the connector broke. Heinrichs sent me a new one even though it was Aquatica's fault. The new board has worked like champ.
At first all of my images were about 1 stop too dark, which I corrected with the in-camera strobe compensation. I adjusted a bias pot on the board and since then exposures have been very consistent, with the usual issues that TTL sometimes gets the exposures a little off if there is a strongly reflecting fish in the frame.
The ETTL and ITTL are somewhat different, so with the D200 things might be different, but I have a buddy who has a D200 who uses it with his Z240 and has had good luck with it.
Overall, I love it and am not a slight bit tempted to go back to manual. Besides, you can switch to manual with the ettl converter installed, simply by switching the knobs on the back of the strobe.