Assuming the water is from a human grade well or filter system, I can’t think of a reason why it would cause diarrhea! While your puppy should be on year-round Heart Worm prevention, sometimes, although rarely, the tablets can cause mild, self-limiting GI upset. There are other non-pill forms of Heart Worm prevention that you may want to try including topical options or a twice yearly injection (which my own dogs receive)!
