I travel internationally several times a year. Make 2 copies of your passport and both sides of every credit card you are taking along. Leave a copy of all this with someone back home that you trust - obviously, as they have your credit card info. On the second copy, back out the last 4 digits of the CC number and the secret 3 or 4 number security code with a heavy marker and then make a copy of that. Destroy the doctored copy and take this copy of the doctored one along with you. Paranoid - maybe, but I'll not trust that someone can't see through the blacking out. Keep this second set separate from your passport and wallet/purse - like back in your hotel safe. Without a passport you can't travel home. Without credit cards you can't pay t stay when your passport has gone missing OR cancel your cards if the are stolen. Also, put a clearly printed list of your hotels and dates, contact person back home, and phone number where you will be reachable while traveling inside every single bag.
When I travel with a partner, we split our clothes between bags too, so that if either of us has a bag delayed or lost we both still have clothes. Be sure to divide by complete outfits though - putting all your pants and their shirts in the same bag is unlike to help either of you!
As for packing under vs carry on - carry what you can't replace, if lost or delayed or broken, and stow the rest. If you are diving with a 4 or 5 star PADI operation or have access to one you can safely rent anything that gets delayed or lost. What you can't replace is your dive log and your computers - dive and personal - and medication and camera gear and jewelry. Carry on only what you can easily carry and remember that small planes = small carry on space. If it won't fit under the seat in front of you, you may have to check it plane side. If you get delayed in an airport of any size for any amount of time, lugging your tons of carry on stuff is miserable.
That's what I do...