Make sure there are no seeds in the diced watermelon.
In the bottom of a cocktail shaker, dissolve the sugar in the lime juice. Add gin.
Put blender jug on base of blender. Add diced watermelon, and then the gin mixture from the cocktail shaker. Cover and blend at high speed until smooth.
Fill cocktail shaker with ice cubes. Pour watermelon mixture over ice and shake vigorously to chill thoroughly.
Pour into two 10-to-12-ounce glasses, top up with the ice cubes. Garnish each glass with a half slice of lime.
In bars watermelon drinks are strained for the sake of appearance but, unfortunately, all the gin and lime flavor is captured in the watermelon pulp, so don’t strain.http://macheads2.net/recipes/WatermelonCooler.html