Common problem. Does it matter? - Well like most things it depends. The biggest potential problem, particularly with older boats is that the internal framework can corrode and the blade part company with the stock. The foam can also become waterlogged and degrade. Assess how bad it is by drilling holes in the bottom and seeing how much fluid comes out. Sometimes the cause is obvious such as damage to the outer mouldings or a split in the join between the two halves or the seal whaere the stock goes into the blade has failed.
As said the water is not as problem in itself but can degrade the strength of the foam and cause corrosion of the structure to rudder shaft.
Certainly dril;l holes in the skin and try to dry out as much as possible. Reseal the holes and fix the intial leak.
Try the whole rudder for strength of attachment to shaft. If it fails then the answer is obvious , you have to open it up by removing one whole skin and rebuild it.
Sorry can't help much more until it is rebuilt there will be an element of concern about the integrity (realistic or not) but it will probably be OK for a long time.
You might think about emergency rudder options to carry. good luck olewill
Depends on the construction, but assuming your rudder has a stainless steel stock then it isnt good at all. In the absence of oxygen ( the water in the rudder will be stagnant) stainless of all varieties corrodes like mild steel.
Every boat I have had bar one has had water in the rudder, usually from where the stock goes into the rudder top.