Several sources say that there is a Thracian river name utus that derives from PIE roots/words *udos or *udes- "water", e.g. this one. The ultimate source might be a book that has an online copy here. The etymology seems ok to me, with a sound shift *d > t from PIE to Thracian that works for at least one more attested word, however few there are for Thracian. But does this really work out from a common sense point of view? I would understand a river with a name like black water or wild water, but simply water without any further qualifier? Isn't that like e.g. a mountain in England with a name of height, rock, or even simply mountain? Does anyone have a good hypothetical scenario how such a name could have developed, or know better attested and more recent similar examples?