Add two cup of watermelon chunks into a blender and puree. Strain the juice through a strainer into a measuring cup. Measure out 6 oz of strained juice.
In a shaker, add the watermelon juice, rum, lime juice, sugar, and ice. Shake.
To make the sugar rim of the, run a lime wedge around the edge of the shot glass and dip the edge into sugar.
Pour from the shaker into the shot glass.
Notes
You are going to have plenty of extra watermelon juice after pureeing two cups of watermelon, but trust me you are going to want to have another shot so its perfectly fine!