I read in another BYC thread that 1 tablespoon of Raw, Organic Apple Cider Vinegar (must not be pasteurized) per gallon of water can be used to treat/prevent worms. I'm thinking this is probably not enough to help your little one at this point and that you need something stronger - I think I...
Like gritsar said, watery poops are part of the normal rotation and may not be cause for concern on their own.
One more thing that occurred to me - have the temperatures been hot where you live? I've heard that diarrhea is pretty common in hot weather because the chickens are drinking a lot...