This is a balancing act. Balancing weight without over-weighing carry-on and check bags. I generally carry-on my camera gear (minus the housing and strobes), regs, dive lights, chargers~batteries, mask, personal electronics, and some personal items (toothbrush, a couple pair of underwear, bathing suit, a shirt). I carry-on a Lowepro backpack, and a roller. I check everything else.
I tend to carry-on expensive items not that scuba gear is not expensive, but bulky items like fins and BC get checked. I figure that I can always rent a BC or exposure protection if need be. If my gear get's lost I'm a bit screwed for the current trip, and may end up without a housing, or have to rent some stuff, but one just has to do what works, and attempt to minimize the loss if luggage is lost. I try to do so by not checking the farm.
Insure everything, get to the airport early, or at least on time. Try to allow plenty of time for connections, and pray!