Volkhard,
The molding wax (toilet washer) or molding clay on the underside of the boat should work. Caveat: I have never tried it myself.
Alternatively you can use a bicycle inner tube to avoid a diver or pulling the boat. However, you said you have limited (easy) access, so this idea may be difficult. Get a section of tube about 18 inches long and carefully slit it length-wise to open (Scissors) it up into a flat piece of rubber 18 inches. Remove the packing and locking nut and slide the away from the stuffing box tube. Water will be start flowing in. Grab a handful if the tube in your hand and stretch and wrap the other end around the the Stuffing box
anchor tube while stretching it. Keep wrapping/stretching the tube up the prop shaft (Baseball bat wrap), sealing off the opening and the water ingress. When you have about 4 inches of the rubber left, stretch it and tie it tight with the beginning free end with a square knot.
The rubber wrap should stop the water ingress either completely or just a few drops a minute. Now you can replace the packing in the stuffing nut at a more relaxed rate.
Once the packing is replaced, remove the rubber tube and quickly slide the lock and packing nuts back and seal the shaft. Adjust the nuts to just stop the drips. After you motor around for twenty minutes or so, readjust to the drip rate of your liking.
If your are concerned that the rubber tube method will take to long or you don't have enough room to do, try it first with the current stuffing box before you disassemble it.
Good Luck.
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Another thing to help with difficult Stuffing Box Nuts: Pretreat the lock nut and packing nut threads with PB Blaster to help loosen them. This stuff works great. You need to be careful applying though as it is very effective and aggressive. What I do is spray a a little in a cup and then use a plastic disposable pipette and directly apply the sprayed liquid to the threads. This avoids overspray, waste, and unnecessary clean-up
DrB