... I'm doing the job afloat and want to work as quick as I can.
Get a bicycle inner tube, cut it open and use it to "bandage-up" the shaft tube side of the stuffing box when you back the nut off to change the material. You should be able to all-but-stop water incursion. The other trick is silly putty in a baggie. Roll silly putty out, put in a baggie, stuff it into the shaft tube side of the stuffing box once the nut's backed off. (Note: The baggie is very