Your mechanic gave you the correct advice Figarow, but there are rabid experimenters who wash them anyway. If you google the subject you will see some reports from those people. Sometimes they succeed, in a fashion, and sometimes they fail.
I've never washed a paper element in water myself. I have washed a completely sludged and oil-soaked paper element in petrol - 8 times IIRC - and it seems to be OK.
There are only two bad things your washed element can do: it can leak dirty air, or it can restrict the amount of air that it passes. You can inspect it to see if it has become excessively porous, but it is more difficult to find out if it has become restrictive. I would assess that by checking whether the engine ran richer with it fitted than without it fitted, but you need a fair amount of experience to make that evaluation.
The best solution is to follow your mechanic's advice.
