Line a baking sheet with parchment paper or a silicone mat. Place the watermelon cubes on the prepared sheet and freeze for 4 hours or until frozen solid (see note).
Meanwhile, in a small pot, bring the water, lemon juice and sugar to a boil. Simmer until the sugar has dissolved. Refrigerate the syrup until ready to use.
In a blender or food processor, purée the watermelon cubes and cooled syrup until thick and slushy.
Serve immediately, adding a wide straw to each glass.
Make sure to space the melon cubes out on the baking sheet before you freeze them. Crowded chunks will clump together when frozen and be tougher to handle. Once frozen, store the melon cubes in a resealable bag in the freezer until ready to use.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.