Rinse the farro well and cook in a large saucepan of boiling salted water. Turn the heat to low and simmer for 25 to 30 minutes, stirring occasionally. Drain. Add the cooked farro to a bowl and drizzle with 1 tablespoon olive oil. Let cool.
Add the mint, parsley, lemon juice, and 1/4 cup olive oil to a bowl of a small food processor and puree until smooth. Season the mixture with salt and pepper.
Toss the cooled shrimp, farro, cannellini beans, watercress, and the dressing all together in a large bowl and serve immediately.