Make the simple syrup. Bring the water to a boil, then add sugar and stir until dissolved. Remove from heat and let cool. Once cool, add to a blender or food processor with the peaches and blend until puréed.

To make the cocktail, rim a glass with coarse margarita salt and fill with crushed ice. Add ice to a cocktail shaker. Add the grapefruit juice, lime juice, tequila, and peach simple syrup to the shaker and shake for 20-30 seconds. Pour into the glass and top with a splash or two of soda water. Stir to combine and garnish with sliced peaches, a slice of grapefruit, and a lime wedge. Enjoy!