Put the sardines and the pat of butter into a heavy-based frying pan and place on a medium heat. As the pan warms and the butter melts, mash the sardines into it. When the butter has completely melted, pour in the water and stir as it comes to a boil. Thoroughly shred the watercress and toss into the pan. Remove the pan from the heat and allow to cool. Puree, and add a dash of fish sauce.