Put water in a big pot, add ginger, chilli, and lime halves, and bring to a boil. For about 15 minutes, cover and let sit.
In a medium pot, add one cup of cold water to the chilli water and simmer until the sugar dissolves.
In a small bowl, combine the syrup ingredients (citric acid, sugar, salt). Keep aside.
Put the lime halves and chilli in a pot with the remainder of the water and bring to a boil. Once the solution has sat for about 15 minutes, drain it.
Slice the watermelon vertically in half using a sharp knife. Using a melon baller, remove the flesh and reserve it.
Add 1 cup of cold water and 1 tablespoon chamoy syrup to the pot. Put the pot on medium heat and simmer until the sugar dissolves, then turn it off.
Add the chamoy syrup to the fruit juice and sprinkle watermelon halves on top. In a bowl, place the watermelon and the fruit juices in the refrigerator for about 10 minutes until the juices are absorbed.
Add ice to a cup of the drink and serve.