marsh: [OE] The immediate origin of marsh is Germanic: it comes from a prehistoric West Germanic. *marisk-, which also produced German marsch and Dutch marsk. This was probably a derivative of Germanic *mari ‘sea’ (source of English mere ‘lake’), whose relatives included Latin mare ‘sea’ (source of English marine). => marine, mere
marsh (n.)
Old English mersc, merisc "marsh, swamp," from Proto-Germanic *marisko (cognates: Old Frisian and Old Saxon marsk "marsh," Middle Dutch mersch, Dutch mars, German Marsch, Danish marsk), probably from Proto-Germanic *mari- "sea" (see mere (n.)).