An Etymological Dictionary of the German Language/Marsch
Marsch, f., ‘marsh, moor,’ ModHG. only, a LG. word. LG. marsch, MidDu. maersche, ‘pasture ground,’ AS. męrsc, m., and the equiv. E. marsh, Dan. marsk, ‘bog.’ Goth. *marisks may be assumed as a derivative of Goth. marei, ‘ocean’ (see Meer); similarly ModHG. Au is a derivative of Goth. ahwa, ‘water.’ Comp. MidLat. mariscus, ‘marsh,’ and some of the Rom. words connected with it, such as OFr. maresc, ModFr. marais, Ital. marese, which may, however, be partly derivatives of Lat. mare.