Heat grill or grill pan to medium-high. Rub the fish with 1 teaspoon of the oil and season with ¼ teaspoon salt and ⅛ teaspoon pepper.
Grill the fish until opaque throughout, 5 to 6 minutes per side.
Meanwhile, with a knife, cut away the peel and white pith of the grapefruit. Cut the fruit into ¼-inch-thick rounds.
In a small bowl, combine the lime juice, honey, scallions, ½ teaspoon salt, ⅛ teaspoon pepper, and remaining 2 tablespoons of oil.
Divide the mahimahi, watercress, avocado, and grapefruit among bowls and drizzle with the dressing. Serve with the bread, if desired.