Place 1 diced ripe beefsteak tomato (about 10 ounces) and 3 sprigs basil (lightly crushed) 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 ingredients to infuse. Stir well and strain, discarding the solids. For serving, add diced tomato and basil sprigs for garnish and plenty of ice.
The infused water will keep refrigerated for up to 2 days.