For travel I use a Lowepro ProTactic BP 450 AW. I'm very happy with it. I can fit my housing, strobes, camera, 2 or 3 lenses, laptop, spare batteries and a couple of other parts. Of course the bag ends up being around 12-14 kg which is super heavy, but the bag is still comfortable enough. I haven't had any problem with airlines in Asia. I like that it appears just as a regular bag, and haven't been asked by airline staff to weigh it.
For boat trips I've been using an AO Cooler 36 for about 8 years now. I am glad that I went with the 36 and not the 24. I don't see that much difference in terms of space taken by my old APS-C Ikelite setup, and the full-frame Seacam setup I now have. Then again the Seacam housing is compact. It's the arms that tend to get in the way. Extra space is always useful for spare parts tupperware, mask, dive computer, torch, towel, etc.
However the zippers are metallic and will get encrusted. I did dissolve those a couple of times with vinegar, but that's a short term fix. I can't move them now and am not even trying anymore. I originally had the thought of using it as a carry-on but never did - yeah it's well padded but still too loose inside, + with all the spare parts, laptop etc... not ideal or feasible.
What I do instead is that I put the AO coolers in a regular suitcase. So, with the suitcase open, I stack the fins first, then the cooler on top, stash the ports and arms in the cooler, then all my clothes. I've even managed to fit the BCD and reg as well in that suitcase. After that there's still plenty of space for about 1 swimsuit and 2.5 t-shirts...