As others have said, waterproofness does exist. The OED lists a 1943 citation:
They offer advantages over certain other cellulose thermo-plastic adhesives by reason of their great toughness, waterproofness and heat sealing properties.
along with another quote from the late 1970s that also references research from that same era:
The old style boot greases were developed during World War II for government use. Waterproofness of combat boots was the main objective.
Interestingly enough, even though the OED had an entry for waterproofness, there was no entry for soundproofness, fireproofness, or bulletproofness. That means, I suppose, I can talk about the waterproofness of my watch, but I'll need to find some other way to describe the the fireproofness of my pajamas, the soundproofness of my recording studio, and the bulletproofness of my limosine's windows. Weatherproofness, on the other hand, is listed as a word.