Place the grapefruit juice in a small nonreactive saucepan and cook until reduced in volume by one-half. Set aside to cool.
In a medium bowl, combine the cooled reduced grapefruit juice, honey, shallots, mustard and garlic and whisk to blend. While continuing to whisk, add the oil in a slow, steady drizzle until fully incorporated. Stir in the basil and season with salt and pepper to taste.
In a large salad bowl, combine the watercress, mango, and 1/4 cup of the vinaigrette. Season lightly with salt and pepper and toss to combine. Serve salad with grilled seafood, simple grilled fish, or chicken, drizzled with extra vinaigrette if desired.