Sheep are terrified of open bodies of water.  Natural instinct tells them that if they fall in the wool gets wet and they sink.   Sheep will drink from ponds/lakes etc only if absolutely nothing else is available.

We've got a sheep pass under the road and the pasture across the road has a lake with a stock pond on the far end.  Even on hot summer days the sheep almost never drank out of the stock pond, they came home and drank.