The bottom rudder shaft bearing is traditionally Lignum Vitae but slippery elm will do as well, but these days they are usually some sort of nylon-polyethene or polypropene compound. Even hard rubber/plastic or Teflon might be found.
Neither bearing is sealed--the bottom one is seawater lubricated, the top one is usually bronze
and is grease lubricated, in my case using white water pump
grease. There is always some moisture which will reach the top rudder bearing through a rudder shaft tube--condensation will see to that even if wave action does not.