There are only 2 occasions to bathe a snake.
1) If it gets poop or other nasties on it.
2) After a bad shed, you can give a snake a warm soak to get the left over shed off. (But there are still better options for a bad shed than a bath)
Otherwise, bathing a snake is ill advised. As LGray said, bathing a snake will strip the essential oils for the snake to properly shed it's skin. The oils acts like a lubricant to slide out of the old skin.
And bathing a snake in really shallow water is safe. She won't die as long as the water isn't high. About halfway of her body.