Bring 2 quarts of water to a boil in a saucepan and add the seaweed.
Simmer for 5 minutes, stirring well.
Add the bonito shavings, and remove from heat.
When the pieces have settled to the bottom of the pan, strain the stock.
Return it to the pan.
Add the soy sauce and salt to the dashi and bring back to a boil.
Add the tofu, scallions and watercress and simmer for 2 minutes, until heated through.
Serve in bowls.