Mar-vaði

As defined by the Cleasby & Vigfusson Old Norse to English dictionary:

mar-vaði

a, m. [Swed. marwatten = eddy], sea water, shoal water (?); only in the phrase, troða marvaða, to tread the water, of a swimmer in an upright position; the word also occurs in Fas. ii. 83 (foot-note, in a verse)—er marvaða mæddu Ránar jóð, but its use there is dubious.

Possible runic inscription in Younger Futhark:ᛘᛅᚱ-ᚢᛅᚦᛁ
Younger Futhark runes were used from 8th to 12th centuries in Scandinavia and their overseas settlements