Translation:By what are you waiting the water or the bridge?
Two problems here:
In English, you shouldn't ever say "where are you waiting at". Just "where are you waiting".
From there, it is totally valid to say either "by the water or by the bridge" or "by the water or the bridge". The second preposition in the second clause is implied, and optional.
I asked a native speaker: Not at all impossible (but rather unlikely!). It is more a follow up question, when there was some exchange before.
I can also think off a parent that is playing with a kid: Where do you see somethin red, by the house or by the car? In Hungarian instead of "Hol" you can ask with Minél (or Miben? or a whole list of possibilities...). But hol is correct in any question where the location is the point.
English just knows five Wh...? questions.
Hungarian handles it differently. There are questions that are exactly the same, but there are also questions that are more specific. This here is like a "Where exactly?" form.
German does btw somethin similar:
There are almost 40 W... words.
Is there any reason why víz and híd decline differently? They both have the same vowel so according to the vowel harmony rules, the endings match right?
Some i's used to be a back (or velar) vowel. Although it's no longer pronounced as a back vowel in modern Hungarian, words with a historically velar i usually retain the back declensions to match. (Or, like "férfi", use both: for example, both "férfival" and "férfivel" are correct.) Other than these exceptions (including híd, nyíl, inni), i behaves like a front vowel, with front-vowel declension. If you want to find out more about this, try searching for "velar i" plus "Hungarian" in English or "veláris (or mély) i" in Hungarian.
adam11151 's question is well discussed by vvsey in the discussion thread pertaining to the English sentence "We eat the bread, you drink the wine.", which is found in the Definite Conjugation" lesson, I believe. But cerulean9's comment is important here.
"By what" is a literal translation. "By where" is still a little literal but slightly better in English.
How the correct answer should look like was explained above by StephenMans2.
My translation, "where are you eaiying, by the water or by the bridge," is better english than the "correct" answer.