I used an older Sea & Sea Y-110a strobe with the Canon G7x ii in a Fantasea housing when diving in Florida last month. The S&S strobes have 3 positions.to move the on/off dial to when you turn it on. The first two are manual, the third is TTL. One of the manual positions would fire and match the camera shutter occasionally! Like every 3 or 4 shots. What is going on I wondered...then I tried the other manual position and it synched strobe to shutter every time.
The GX7 ii will also do TTL, but the issue is camera battery life. To get TTL, you have to leave the camera flash on auto and that means full power even though all the camera flash does in the housing is signal the strobe to go off. If you are only doing one dive and not taking that many pictures, it's not an issue. If you plan to do several dives that day and don't want to change batteries or will do a lot of shooting on one long dive, you can set the on camera flash to manual and lowest power. That's still adequate to fire the strobe and saves battery life. That's what I used with the S&S strobe set to manual.