Water pump - replace regularly, especially if you have been operating in the shallows and stirring sand. Water pump impellers deteriorate when not used for lengthy periods. Fuel filters and lines - filters become clogged and lines can harden with age and exposure. Propellers - the bushing of the propeller can fail especially if it has hit sand or rocks. Caught line will also cause problems.
Some older models have a shear pin to keep the propellers in place.
Carry a spare shear pin when appropriate and a spare propeller. Spark plugs - new plugs can cause trouble, but old plugs can break down completely. Gearbox oil - snagged fishing line on the drive shaft is a common cause of leaking gearbox seals. Water in the gearbox will eventually cause it to fail. Regular oil changes will prevent this.