Bring water and honey to a boil over medium heat. Remove from heat and add tarragon leaves. Allow leaves to steep for about 3 minutes. Remove leaves and allow syrup to cool.
Divide vodka, St. Germain, honey syrup, and grapefruit juice between 2 tall Collins-style serving glass. Add a slice of grapefruit for garnish if desired. Add ice, then top with sparkling water and stir before serving.