definitely feather out the edges of your swamp layer where it meets the ground layer. In addition to feathering I would go in with a small hard brush eraser at about 20% opacity and cut into the edges in places to rough it up some.
This gets a more realistic look or artsy look but you will lose some clear delineation of "now you are entering the swamp" it would be more gradual and broken up. So it depends on how you want your map to read.