In terms of whitetails i haven't seen to many at a tank, but when the snows on the ground I have seen them take mouth fulls and licks. I believe they drink however because they like green foods also believe like others mentioned they get hydration from foods (when plants are still living).

Now mulie's I have seen at tanks, in fact locating such sources is of more importance when I'm hunting/scouting them. I believe part of this to be diet related as well as they inhabit sage/buck brush which isn't as lush as whitetails grounds.

Not scientific rather observation based.