In a high-speed blender, blend together the frozen watermelon, tequila, triple sec and lime juice. Add a pinch of salt.
To make the sugar rim, place caster sugar in a small, shallow plate. Moisten the rim of the cocktail glass with a slice of lime then dip the rim into the sugar.
Pour in the blended margarita. Garnish with a slice of lime and mint leaves. Serve straight away!