bit of a silly question really.. but i've never been told and i'm not sure! Does it matter? I would have thought it didn't matter but I could be wrong... Lemme know!
In a natural environment horses would drink from rivers and streams so from the ground. However nowadays you get a lot of water drinkers which are placed on a wall and never done any harm!
As an aside to this point,I have seen horses apparently go off their water and food after a flu jab in the neck . Truth was it hurt to put their necks down . Simple solution ,raise the buckets. As with most horse questions ,there is no hard and fast answer and emphasises how observant we need to be to the individual circumstances.