If the top nut is close to the waterline then the water inflow won't be much, as there is no head of pressure driving it.
Undo the nut, and raise it up out of the way. Use a drill bit to confirm the size of the gap between the rudder shaft and the housing, then get some new packing material the same size as the drill bit that was a tight fit. For a rudder I don't think you need to go to an expensive type of packing. The inexpensive and common teflon-PTFE impregnated flax will be fine. Prepare several rings of it.
Remove the old packing - a steel hook probe is useful for doing this. Then start putting the rings in place, offsetting the end points of the rings from one another. Tighten the nut, but don't over tighten it. Check it after a trip, or a bit of rudder use, and tighten a little more as required to stop drips. You might have to add another flax ring if the nut runs out of travel. But once it has bedded in after a few trips it will not leak at all.
There is no need at all for any 5200 !