Slice watermelon into cubes, removing rind and seeds. You'll need about 5 cups of cubes. Put 4 cups watermelon cubes into blender and blend until smooth. Pass watermelon juice through a fine strainer to get a clear watermelon juice. Transfer juice into a large pitcher, add 1 cup of saved watermelon cubes.
Wash and dry cucumber, then cut into thin slices and add to pitcher.
Cut lime in half and press out the juice, add to pitcher. Wash lemon and cut it into thin slices, then add to the pitcher.
Get a few stems of fresh mint, wash and then add to pitcher. Add water until pitcher is full and give a good stir. Refrigerate before serving or add some ice cubes to cool down the drink.