In a small saucepan over medium high heat, make a simple syrup combining equal parts sugar and water, cooking it until the sugar dissolves - I used 1/2 cup sugar and 1/2 cup water and cooked it for 4 minutes. You could also buy simple syrup or use less if you don't like sweet in drinks.
Ball or cut the watermelon, cucumber and lime. Put watermelon and cucumber into a pitcher, and squeeze lime wedges into the pitcher as you put them in.
Fill the pitcher 1/2 way with ice, then pour in simple syrup and 2 cans of LaCroix. Stir. Refrigerate for 2 hours, or overnight (lasts in the fridge with plastic wrap over the top for one week).
To serve pour a glass 1/2 full with contents of the pitcher, and fill the remainder of the glass with LaCroix