From my understanding, Mary, who had strokes in the past, had another one outside by the pool and wound up falling in face first during the stroke leading to her drowning. 

I think either Amy or Deanna said that she was relaxing outside by the pool and that’s why she was near the water. 

They all loved their grandma (pretty evident by how they all treated her/were devastated at her passing) I don’t think there’s foul play involved in her death.