This is how the couplings looked when I began taking them apart.  The mild steel coupling, which had never been painted, was coated with surface corrosion which, thankfully, didn't freeze the setscrews or flex coupling bolts in place.

Here is the Yanmar flexible coupling back in place.  I painted it red to match the newly painted coupling, and because this was the color engine paint I had a round.  Plus, I like to have the couplings stand out from their surroundings anyway for easy inspection.  You can see some of the blue waterproof grease I applied to the threads of the studs.  I did this on all eight studs--four that secure the coupling to the transmission flange, and these four, which secure the shaft coupling.

I greased the end of the shaft heavily before sliding the coupling on.

Here is the reassembled coupling and shaft.  Compare with the top photo.  There's more of the water proof grease on all the mating surfaces--inside the coupling, on the shaft and keyway, on the setscrew threads, and inside the nuts securing the coupling to the flex coupling studs.  I also greased the threads on the packing nuts to make those easier to break free for adjustment.

Before I reinstalled the propeller, I applied more of the grease to the shaft taper, keyway, and threads.

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