The script does not specify what sort of paper is used, but I always assumed it to just be a regular piece of tracing paper.
Tracing paper is a type of translucent paper. It is made by immersing uncut and unloaded paper of good quality in sulfuric acid for a few seconds. The acid converts some of the cellulose into amyloid form having a gelatinous and impermeable character. When the treated paper is thoroughly washed and dried, the resultant product is much stronger than the original paper. Tracing paper is resistant to oil, grease and to a large extent impervious to water and gas.
But historically, most rubbings are done with a paper that is very similar to Butcher paper which is also very resistant to water.
Interestingly, the script makes specific mention of the rubbing being "soaked", though it does not indicate that it is damaged at all.
CLOSE ON THE WATER-SOAKED PAPER IMPRESSION: The rubbing taken from the shield of the Grail Knight.