I wouldn't concern about getting a waterproof wallet. The phones are designed for the real world and can take moisture. I have not used a waterproof wallet for years and have had no problem, even when the screen is steamed with moisture. The evaporated water from your body does not contain salt, that is left behind in the evaporation process.
I would consider heat more of a problem. Wallets retain the heat that cell phones generate, and heat is the enemy of any electronic device. I don't suppose any company advertises a heat-conductive phone wallet, but that would be better for for protection than water proofing.