Combine 1 cup of the sugar, 1 cup water and the watermelon in a small saucepan. Bring to a simmer over medium heat and cook for 5 minutes, mashing the fruit gently. Strain through a fine-mesh strainer; discard the fruit. Repeat with the blackberries and blueberries and the raspberries and strawberries. Cool the three fruit syrups completely and refrigerate until ready to use.
To serve, add 3/4 cup club soda to tall glass filled with ice. Add 1/4 cup fruit syrup and stir with a long spoon or straw.