My emergency toolkit is a combination for camera and diving gear (two cameras, housings and diving gears (me and my wife)):
#1.: Small sized toolset in a pouch (goes with check-in lugagge): combination pliers, side cutter, two sets of Phillips and Slotted screwdrivers, two adjustable wrenches, and tweezers, set of Allen keys (metric and imperial).
#2.: Spares (dumped in camera bag and check-in lugagge): Spare flash, two spare fiber optical cables, spare small camera flash (in case the optical trigger fails), spare complete regulator set, spare mask, spare ribbons for mask, knives and fins, spare diving computer, spare O-rings for housings, (dome)ports and flashes, six CR2450 (for Nauticam housings, flash trigger and diving computers), four spare Eneloops (AA and AAA each), spare Nauticam grease, cabel ties of different size, spare accus for cameras.
I have needed so far almost everything on several occasions and it would not have been available at the location. No spare housings and cameras, when a failure happens, my wife and me share one rig during the dive (the diving is then more like in the old days, before we started UW-photography, and much more community experience and communicative...
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Wolfgang