In a large pot, combine the ribs and enough water to cover by about 5cm. Bring to a rolling boil. Drain the ribs into a colander over the sink to get rid of fat and scum and return them to the pot.
In the meantime, trim the watercress, separate the leaves and stems and chop the stems into small pieces.
Add about 2 litres of water to the pot followed by the watercress stems, dates, and ginger. Bring to a boil over high heat. Reduce the heat to medium and cover. Simmer gently for 45 minutes to 1 hour, or until the ribs are tender.
Add the watercress leaves and simmer for another 10 minutes, or until the leaves turn dark green. Stir in the salt and sugar. Serve immediately.
The Cantonese believe that watercress should only be added to water that has been brought to a boil. Otherwise the watercress will turn bitter.