Divide watermelon among two tall glasses; muddle the melon.
Fill the glasses with ice. Add 1 shot of vodka to each glass, if desired.
Add two tablespoons of lemon-basil simple syrup to each glass.
Top off with Pellegrino or club soda; stir.
Garnish with basil and sip.
Lemon-Basil Simple Syrup
Stir water and sugar in small saucepan over medium heat until sugar dissolves. Remove pan from heat. Add basil and lemon zest. Cover and cool. Stir in lemon juice; strain. Syrup keeps about 1 week in the fridge.