Place 1 thinly sliced English cucumber in a pitcher. For still infused water, add 2 quarts of water. For sparkling water, add 1 quart of seltzer during prep and a second quart just before serving. Refrigerate 2 to 4 hours to allow the cucumber to infuse. Stir well and strain, discarding the cucumber. For serving, add fresh cucumber slices for garnish and plenty of ice. The infused water will keep refrigerated for up to 2 days.