Arthur Custance writes: “It is a curious fact that demons were notable for their physical strength, and it is also a curious fact that they had for some reason a fear of water. The instance of the Gadarene swine could conceivably be a case where the Lord permitted the evil spirit to enter animal bodies only because He knew that the animals would react to rid themselves of those who possessed them. It is a curious fact, too, that the Lord should have said that when the evil spirit has gone out of a man, he goeth through waterless places [so the Greek] seeking rest.” (Matthew 12:43).
(www.custance.org/Library/Volume7/Part_V/Chapter1.html)