I do a lot of dive travel and my gear bag ususlly weighs in at about 30 kg! I have "real" dive gear bag because I like the side pockets for fins, and because I like the wheels. Pockets keep the fins from getting damaged and wheels keep Q from getting damaged, LOL. Here's what I take:
--wetsuit, rashguard and any other neoprene stuff I need (hood, core warmer) for the water temps I'll be diving in
--BCD, and wrapped in the BCD in their own carry bag for added protection, my regs
--save-a-dive kit, booties, torches and batteries/chargers, SMB + spool
--mask and spare mask, camera housing, camera/housing supplies (battery chargers, silicone grease, spare o-rings)
--fins
I put a lot of these things in a mesh bag I can use for daily hauling of gear when I'm doing land-based trips, and then put the mesh bag inside the regular gear bag.
If I'm doing LOB diving, I try to take only the mesh bag aboard the boat and leave the big bag in the office of the dive op. Also for an LOB, I often take a small pillow and little blanket for sleeping on the deck (sometimes, if I'm staff, I don't get a berth) and clothespins to clip things to the rails.
In carry on, I take my camera, strobe, dive computer, laptop, c-cards, toiletries, and clothing. If I'm teaching, I take a satchel with all the stuff I need for the course.
It's a shedload of stuff!
BTW, most airlines have a free allowance for golf bags. I know some divers who put all their gear in golf bags to get this allowance. Only a few airlines have extended this to dive gear, and those airlines usually serve dive destinations. (Just like some airlines in the US and Europe let you take skis at no extra charge.)