A taper fit such as used on prop to shaft is a "machine dry fit" & is intended for metal to metal so it won't slip. The key & keyway are not really reqd to prevent slippage as long as the taper fit is held tightly together by the prop nut.
The key & keyway on a prop shaft are there as a secondary safety
in case the taper fit fails, & the key would not last very long if it was subjected to the "loose" prop beating back & forth on it.
Check out a good quality drill press that uses morse taper spindle-no keyway & you can remove/install bits with a light tap.
Taper fit is used heavily in industrial machinery,without keys,where items must be removed/replaced often.
A good taper fit of a prop requires light polishing with a fine lapping compound,cleaning thoroughly,& dry fit installation
"fitting a prop to tapered shaft".
If you lube it or coat it with anything,you decrease/destroy the strength of the joint.
Perhaps what your mechanic
used was "mechanics blue" paste to check for perfect contact of prop & taper. Same stuff used to check gear
tooth contact in geared machinery,as well as crankshaft journal to bearing insert contact in engine
The bluing grease is then washed off thoroughly after fit is attained.
IMHO,a bronze or SS prop on a SS shaft should never be taken apart unless you need to replace cutless or other components,& then I would leave prop on shaft if possible.
There cannot be any appreciable corrosion
in a properly fitted taper,since it is water
Try tightening the adjusting nut on a taper fit seacock & see if you can turn the handle.