I dive solo, so I have redundant everything.
Extras:
Day-trip boat Dives: Nothing, If something is wonky I'll rent what I need
Shore Dive near home: The full tool box and spares kit.
Resort or LOB: Adjustable wrenches, Hex keys, Wire cutters, several pliers, bit driver and 12 bits All Stainless Steel (thank-you McMaster-Carr), O-Ring kit, Tribolube, Spare HP, LP, Inflator hose, Spare inflator assembly. Spare regs. (12lb SAD kit)
Objetive: Don't go near the water with questionable equipment. Don't miss a dive.