To prepare simple syrup, pour 1/4 cup (60 mL) boiling water over the 1/4 (60 mL) cup sugar. Stir until sugar dissolves. Set aside to cool completely.
Mix salt and coloured sugar (use coloured "decorating sugar" from the baking aisle); spread onto small dish. Cut lime wedge and rub onto rims of 8 shortball glasses (about 1 cup /250 mL each). Dip rims in salt/sugar mixture. Set aside
In blender, blitz frozen watermelon, tequila, lime juice, orange liqueur and simple syrup until just slushy. (Notes: To freeze watermelon ahead, line a baking sheet with plastic wrap, arrange cubes in a single layer. Freeze overnight until firm. For non-alcoholic version, substitute tequila and orange liqueur with equal quantities of carbonated water in lime or orange flavour.)
Divide watermelon slush into 8 glasses. Top with carbonated spring water. Garnish each glass with lime round, if desired.
Transfer frozen watermelon cubes into freezer bags to store.