there's still no reason to let it get wet
"The nonenzymatic hydrolysis of cocaine to BE can be slowed or eliminated by acidifying the sample and storing it at 4°C" - Cocaine will hydrolyse slower if it is kept at a proper pH and kept cold (ref: 10.1111/j.1365-2710.1978.tb00096.x). Even so it does not last forever.
If you just throw some cocaine into tap water and let it sit it will degrade much faster. Also cocaine kept moist over weeks or months will do the same.
If you are going to spend money on cocaine I see no reason that you shouldnt take some simple steps to keep it "fresh". if on the other hand you don't mind intentionally weakening your blow, well, more power to you then.