To write "major water problems facing the world" or "challenges and
opportunities facing low- and middle-income countries and their
citizens", seems to me to switch the roles. Is it OK to use "facing"
in this way, or is would the only correct usage be "faced by" in these
How would "a water problem" face anything?
Something or somebody might face a water problem, but you can't interchange the subjects and retain the same meaning as a matter of perspective. To face, confront, negotiate, or any other similar "activity" are best understood as activities done with intent towards a perspective or orientation.
To face anything, it means the primary subject adjusts their orientation to be more aligned with the secondary subject. The secondary subject does not need to do anything, or even be aware of what the primary subject intends or has done whatever they were subjected to (being faced, being looked at, being mocked).
However, because of the concept of allusion, a certain way of creating a figure of speech, you can at times suggest that things that normally do not possess an ability to demonstrate perspective or orientation with intent, might do so through alluded embodiment. Such things are now "living, thinking, purposeful entities." Suggesting something though isn't a matter of fact. Water can not nor will ever be able to bring suit against people for the problems we have allegedly inflicted on it. People might be able to, but of their own accord, and not at the behest or behalf of water.
Literally, it would not make sense in the slightest for inanimate objects or concepts to "face" a subject. Figuratively, sure, go ahead, but realize, it may not be well understood or appreciated in the same manner by the audience. It will likely translate with difficultly, too, if that is ever a concern.