First rope up the rudder using the hole in the top, to avoid losing it! Then the shaft needs to be separated from the rudder. A large cold chisel, mooring pin or other wedge shaped object has to be forced in to the slot in the rudder just under where the stock taper fits into the taper socket in the top of the rudder. Hit the wedge firmly with a hammer and the taper will part. The stock can then be lifted out of the bearings (it's heavy) leaving the rudder in the bottom bearing on the skeg, and supported by the rope. It can then be lifted out of the water by the rope.
It can be done in the water, but depending on how tight the taper is, it may be difficult. Out of the water is easy.
Richard