"In what is there a lot of water?"
Translation:Miben van sok víz?
No, it isn't.
Question words are usually in the focus, so they should be followed by the verb ( if the verb has a prefix, it should separate.) The two exceptions that come to my mind are miért (why? requires a focus, but it's not necessarily the question word) hogyhogy (how come? doesn't actually require a focus.) Also, if there are multiple question words, they don't have to all precede the verb. Indeed, "Ki látott kit?" (Who saw whom?) is correct whit all four word orders that don't have látott in the front.
EDIT: Also also, just to answer the next logical question: Sok víz miben van? is correct.
The multiple choice exercise offered "Mikben van sok viz"
If I already know that it is plural, why even ask in what? I seem to know the what when I know that there are more than one.
Wouldn't "Melyekben" be then more logical?
And shouldn't it be vannak?