The Old Norse word vatn
Word vatn
v a t n
Meaning water
See also Proto-Indo-European: *wódr̥/wudr̥ ‘water’
Hittite: ua-a-tar ‘water’
C. Luwian: wa-a-ar-ša ‘water’

Other words that might be of interest
frost ‘frost’, regn ‘rain’, marr ‘sea’, vøkr ‘wet, moist’, væta ‘wet, sleet’