Crowley can feel the holy energy of the water before he even sees it, and that is the only reason he is able to dodge the stream as it comes at him full force. Even one drop of holy water would obliterate him completely.

"Oh fuck!" Crowley cries out, holding his hands up to keep them away from the water, but leaving him totally vulnerable to the beasts.

He knows his fire doesn't do anything, but maybe his imagination can. He imagines the ground beneath the beasts is suddenly thin, like quicksand, pulling them down.