To make the lime sugar, mix together the lime zest, sugar and a pinch of salt on a plate. Rim your cocktail glass with the lime sugar.
To the cocktail glass add the watermelon cubes, fresh mint, lime juice and sugar and muddle together, squashing everything to release their juices (you can use a muddler or wooden spoon to do this). Once the watermelon is fairly well broken down add the rum and ice and stir.
Top with a splash of sparkling water or club soda and garnish with a watermelon wedge and/or fresh mint.