Wow, that is a tough one.
A snake's skin, like ours is designed to keep the elements out. Thus, the only way to get water inside is via the mouth. I have no idea if a vet could do an IV or not. I don't know how difficult it is finding a vein on a BP.
But I digress. It seems that most of the water comes from the food. Many here say they never see their snake drink. I do. What I'm getting at is maybe there is some temporary kidney problem going on here in the way of a bug. Or maybe the food source was just low on water.

The main thing, just keep doing what you are with the water. And when you go buy a rat at the pet store, check out their water supply.

Good luck