Wiktionary gives 'vanduo' as modern Lithuanian for 'water', and 'udens' as the corresponding Latvian word. In both etymologies the same Proto-Balto-Slavic root of *wondor is given.
However, at least to me as an etymology hobbyist, the start "ud" of the Latvian word 'udens' suspiciously looks like the zero-grade form *ud- of the PIE root *wed-
Could it be that the original *wondor was still ablauting in some way and had a zero-grade form, and then Lithuanian kept the o-grade form, whereas Latvian kept the zero-grade form?
If yes, how this zero-grade form could have looked like? If no, how do you arrive at udens from *wondor?