Prepare the simple syrup. In a small saucepan, add sugar and water. Stir to combine. Bring mixture to a simmer over medium heat until all the sugar is dissolved. Remove from heat and allow the syrup to reach room temperature.
For the watermelon puree, place watermelon into a blender and blend at high speed until smooth.
Prepare the margaritas. Rim 2 glasses with salt and add ice. Combine tequila, watermelon puree, lime juice, and simple syrup in a cocktail shaker and shake with ice. Strain mixture onto the prepared glasses. Garnish with lime and watermelon slices. Salud!