My Spare parts kit for my Camera is most replacement parts. My general rule of thumb is anything that can easily be knocked off or that I could potentially drop and lose when assembling the camera gets a spare.
So In my kit I have O-rings/grease, batteries, sync cord, replacement battery caps for my strobes, replacement vacuum seal cap, spare pump, screws for my nauticam housing that are used to attach the handles, and the necessary tools to assemble everything. I also carry a spare camera body and a flash to use on land without having to open my housing, but they also double as a spare if the camera or flash trigger fail.
