DrAlex

hi ErolE
You can solve this problem using the system flash in the housing.
I have DIY housing for Canon 580 EX 2 and using control via the camera menu of Canon 5d mk 2.
the second curtain sync and high-speed sync work perfect.
I can use multiple flash also

Hi all.

I do not wanna to discuss in this thread is needed TTL mode underwater or not. If that, I shoot in manual mode, the last 8 years.
I carried out experiments with system flashes Canon 580 EX II with Canon 5DmkII in a custom housing.
The first flash was managed and synchronization by electric cable (6 wires), and the second flash via an optical fiber (I use TosLink audio cable). Flashes as Sea&Sea YS 250 Pro and Inon Z240 did not work in TTL mode. They fires only in Manual mode with the optical synchronization.
Next step was when I use the system flashes - Canon 580 EX II and Nissin 866 pro mk II.
I was able to flash  in e-TTL mode as a clear mining on the master flash on the cable wire, and on the remote flash unit via optical cable. Custom Housings for system flashes was without any control buttons.
I operated of all modes of the two flashes via the camera menu.
It was really great.