@sunnyboy
I just tested TTL top side and it works. I don't see why it would be any different during a dive.
To do this, both camera and strobes were set to auto. Selected an area of low light, took the shot with strobes off and it came out underexposed. Powered up strobes and it turned out properly exposed.
I'm using INON Z240s. I think you said you have Ikelite. I'm not sure if Ikelite supports TTL through fiber optic. I've moved away from Ikelite some time ago, so can't test or properly answer for sure.
Cheers,
Ricardo
The Ikelite DS125 (and DS51 I also have) have a wired bulkhead, so only work with wired cords. To get TTL you have to have a connector that supports all 5 pins (fire & quench primarily). My Ikelite G16 TTL housing has the control board to do this, but the Nauticam does not.
I do have an Ikelite optical trigger that I got used. It allows you to control any Ikelite strobe by screwing it into the strobe bulkhead. It will fire the strobe on any optical signal, so is mainly used for remote triggering (back-light, etc.). It does have a plastic fitting to couple to an optical cable, but I don't know if it supports TTL - I suspect not as it's pretty old tech. I'd also have to get an optical cable as I don't have that either.
My best option is to eventually go fully optical with cables and something like your Inons. Those are pretty awesome strobes anyway, so a good way to go in the future.
Cheers,
-Richard