You can cable tie together a camera housing if it breaks.
You can use weight belts instead of gaiters if you leave your gaiters behind. Or duct tape. My buddy used to have a drysuit that was too loose around the legs and hated diving without gaiters so used the weight belts when he forgot his gaiters once - he said they worked better than actual gaiters
You can trim your mo with shears underwater if your mask isn't sealing properly.
Socks under booties are warmer than socks that dive shops sell. Much cheaper too! You can get enough pairs of normal socks for the same price as dive shops so you can have dry feet always between dives.
Marking off spools each metre and then doing a different colour line at each five metre mark (i.e. each metre has one black stripe, at 5m you have one red stripe, at 10m you have two red stripes and so on) is very handy for navigation and measuring viz.
Carry o-rings for every bit of your gear that has o-rings. Tank valves, regs, computers, spg, torches, camera housings, valves, hoses, etc. Nearly all of my problems are caused by o-rings and they are hardly any cost at all to carry your own supply. Also carry a shifter and allen keys and a pick