Corrected entry: In the episode "P3 H20" we learn that Patty died because of the Water Demon, which is why Prue is afraid of water. However in this episode we are told that Patty was killed by the Sea Hag, and that's why Piper develops her demon fear upon becoming pregnant.
MeganP23
6th Sep 2016
Correction: Paige actually mentions the water demon. Saying that the Sea Hag reminds her of it and that it kills in a fascinating way. That is when Piper states that she knows about the water demon because it killed their mother.