Maybe some bright spark might have ground flats on the shaft for an open ended spanner?
Big Stillson wrench(es) can be used to grip a shaft but will mark it.
If the prop is still on you could thoroughly rope
it to cleats
and winches port & starboard on deck
with rags to protect the hull
burns. If the P-bracket's strong enough you could use that.
Presumably if the propshaft is rotating freely it should be easy enough to avoid damaging the transmission?
A three-jaw puller and a tap with a mallet while it's tensioned will usually break the taper.
I'm sure you'll have checked the handedness of the nut so I won't even mention it.
There are probably better ways to do this but I can't think of any yet.
later edit: slotted shafts so we could use morse taper type drift keys would be nice.