Lithuanian and Latvian words for water deriving from different grade

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?