Does he have access to water all the time, or was this after he went from no water to a full water dish? If he's been dry, you should make sure to get him a full and fresh bowl of water every day to avoid him gorging himself on water (I don't know if it's that bad a thing for BPs, but I have had a juvenile beardie guzzle water until he started spitting it back up).

Is your tap water especially chlorinated, or high in other chemicals? All tap water is different, but if it's safe for you to drink it should be safe for your snake as well. I don't dechlorinate or treat my tap water in any way and I haven't had issues with any of my animals (10 snakes, a bearded dragon, a dog, and several rats).