The kidneys are responsible for maintaining the Calcium/Magnesium balance in the body. Weak kidneys release more magnesium than they should, notably in diabetics.
Sodium is what causes water retension and is hard on the kidneys. Epsom Salts have no sodium in them. They are Magnesium Sulfate (MgS4).
That said, drink lots of water before the ES mix to have enough water in the body for kidney function as well as the bowel elimination of excessive Magnesium.