Have seen it attempted once on a Dutch Salvage tug I was on. It was a Greek vessel in Ballast. After several days trying we towed it into the Azores to get done. I personally think even using conventional lifeboats at sea if vessel had no assistance it would be virtually impossible.We had perfect sea conditions etc but couldnt complete operation.
Not quite at sea but did witness a ship having her prop changed while on the mooring in Colombo Ceylon mid 60's. She was pulled hard down by the bow and away they went........it was a large ship........no camera in those days...............and my how I regret it.
Not exactly a prop change, but when I was on the Border Pele in 1970 we were in the Persian Gulf and had some sort of a problem with the prop shaft, a leaking gland or something. I know we stopped the ship and ballasted her down by the bow to get the stern high enough for the engineers to get the work done. It was a very strange sensation being on board ship with her in that attitude.
I was only a lowly deck apprentice so I didn't get to know the full details of the work they did.
Have just posted some photos in the Tanker section, of a prop being removed from the Emma Maersk in the Sembawang Naval Dockyard in Singapore during 1970. The size of the gear used shoud indicate the problems that would be faced trying to do this at sea.
We repacked the stern gland off Bijouga Breakers on Hector Heron, straight out of drydocking at Swan Hunter's - somebody made a bit of a cockup there ! As we were light ship, and ballasted well down by the head, it caused several passing vessels to come near and see if we were OK ! We had to do it again when fully loaded later that trip, but this time went into Mombasa as a port of refuge and had the local divers down.
An unexpected week in, at the buoys directly opposite the boat landing. Went down very well with all on board !