Blend watermelon cubes until smooth. Add light rum and 1/2 cup ice. Blend.
In a small pitcher, combine mint leaves, sugar, and fresh lime juice. Using the handle of a wooden spoon, muddle the ingredients to release the oils from the mint. Do not muddle so that the leaves fall apart.
Fill four tall Collins glasses with cube ice. Add watermelon mixture to the pitcher. Stir. Divide the mojito amongst the glasses. Garnish with lime wheel and mint sprigs.