Can be done
This last spring I had to replace the bronze stearn tube bacause the hose slipped off the old worn and corroded tube. Did it in the water with an experienced diver. He showed me how to stop water from entering the boat through the stearn tube/shaft through hull. Take a thin cloth and rip it into strips. Then from underwater push these thin cloth strips into the gap between the stearn tube and the shaft with a thin screwdriver. The water pressure pushing into the gap holds the cloth "plugs" in and keeps the water out. As was said earlier you with have to disconnect the shaft from the transmission to do this job and pull the packing gland off of the shaft. Best of luck.