Esta doesn't particularly realize that Merrin is regarding freshwater as a valuable resource; she didn't mention that part to him.
He will Create new Food and Water in their appropriate receptacles, filling up the Water receptable to the top, creating enough Food for three people of whom he himself is one, and changing up the Food's colors and shapes so there's no possibility of confusing it with the last set of Food. Then he'll throw the old Food away from their sanctum, as far he can heave it, before it starts to very rapidly rot.
And then, before it's time to pray, Esta will try to put his mind into a state that Asmodeus might maybe be able to see better, from far away.