An Etymological Dictionary of the German Language/Annotated/Haff
Haff, neuter, ‘inland sea, gulf,’ a Low German word, originally ‘sea’ (generally), which is also the meaning of Anglo-Saxon hœf (plural heafu), neuter, Scandinavian haf, neuter, Middle Low German haf; the Upper German words, Middle High German hap, habes, neuter, and habe, feminine, which correspond in sound, also signify ‘sea,’ as well as ‘port’ (see Hafen). As we need not assume an originally difference between the words for ‘harbour’ and ‘sea,’ and since in any case the meaning ‘harbour’ is derived from the signification ‘sea’ — the converse would be hardly possible — the usual assumption mentioned under Hafen (2.), that Hafen is literally ‘receptacle,’ is quite problematical. Hence Hafen may probably be explained by some such word as ‘marina,’ in the sense of ‘statio marina.’ The connection of Anglo-Saxon hœf, ‘sea,’ as ‘heaving,’ in the sense of Latin altum (‘high sea’), with heben (root haf, pre-Teutonic kap), is not impossible, though scarcely probable.