I have previously asked a similar question about wear in the rudder stock. My present theory is this.
When on the hard the rudder hangs from the (worn) top bearing showing sideways movement at the lower.
When in the water the rudder has positive buoyancy, pushing the rudder against the white plastic ring.
The top bearing then floats, on mine to the extent that water in the access hatch runs through the bearing and down the rudder stock, subsequently rusting the oil seal type seal in the rubber tube.
If I could turn the adjusting collar I would try, but at the moment no amount of pressure will turn it, due I think to the locking screw having been tightened too much and the collar deformed where it is split.
If it would stop raining perhaps I could have a serious look at it.