Flim,
It will be very difficult to do so in the water. Part of the changeout is uncoupling
the shaft from the coupler flange at the tranny, all the while finding a way to block
outside water from coming thru the shaft log because you will need to push back
the shaft for room to install say, a PSS shaftseal. Then there is shaft cleaning & prep.
Impossible no, difficult yes. As an option, have the yard haul your boat & leave it in the sling
while you do the switch. You should save money because they won't have to block it up on stands.
After all, it's a boat $$. I tell folks that in my next life, I'm coming back as a firstmate only. It's
alot cheaper, good luck.