I always try to carry my camera gear with me into the cabin. Usually with a backpack and largish waist bag (Think Tank). This worked fine with a Sony RX100 in a Nauticam and dual strobes. But it no longer fit when I went to a DSLR rig. (Nikon D810 and Nauticam housing, with 105vr macro port and 16-35 dome port). The problem is the dome port, which is 230mm. It's just HUGE in terms of packing. (Not light to carry either). I can get all the rig except the port into my carryon, but the port has to go into my wife's carryon, or checked luggage. As my next dive trip will be me alone, I'm going to have to put the dome into a checked suitcase, and I'm looking for some sort of protective hard shell to hold it inside the suitcase.
I can't stress enough how large that dome port is. It's made me wonder if I should have gone DX instead of FX underwater, just to have a smaller dome port. I literally cannot fit both the housing and dome port in the same carryon, even with no other gear.