I spent couple years shooting with 5dmk2 without any TTL because the old Mathias TTL converter that I had working with my 5d didn't work with 5dmk2. I really missed that feature for few months, mainly for fish behavior, but I we are capable to adapt and change our mind to shoot without that making the adjustments manually.
Since Feb this year I upgraded to 5dmk3 on Aquatica housing and started to use the YS TTL Converter/C with couple YS120. I am really glad to have it back. It is very reliable.
The only cons I have about that TTL converter is related with appertures over f/22 where we need to increase the light compensation using the dial in the converter, but it doesn't vary, I just got in mind the compensation for f/22, f/25 and f/32 and it became automatically for me.
I made my decision three months ago to Aquatica for my 5mk3. I didn't dive with Nauticam, but had the opportunity to get both it in hands before decide. The Nauticam looks nice, had lot of engineering onboard, but I felt Aquatica more solid/strong and the simplicity of the commands inside made me believe it should be more reliable and easy to repair.
But it was my perception, I think you should locate a near reseller and taste both housings...