Just passing on good experience...
After trying to figure out how to carry the compact digital camera, camera housing, video camera, video housing, to avoid the dreaded Pelican theft, AND the reg, I found that my photo backpack swallowed it all up.
I was able to put all the camera gear, camera housing as well as the octo into the photo backpack, while the video housing fit compactly in it's own little pack. That made two carry-ons out of 4, and I keep the video housing with me. Necessitating lugging the thing, but eh, we're divers.
This Pelican theft thing is tough to trust. It would be much easier to let them schlep it, but I can't see losing the thing I've worked so hard to bring.
YMMV