For what its worth
I Carry
Engine spares, Belts, Spare impellor, Oil and Fuel filters, fuses. Engine and stern drive Oil, engine coolant.
Boat spares
Lamps for Nav lights, deck lights, fuses for bilge Pp's electrical tape, various crimp on fittings, various lengths sizes of hoses and jubillee clips. cable ties, rubber bungs, rags
some sealant, General purpose grease, spray oil, and anti moisture spray
and of course most important a good collection of tools
No point in being a Marine Engineer and carrying half a ton of spares if you do not have tools available.
Hope that helps
Charlie, [i am sure as the boat grows older so the list will become longer.]