Preheat a grill to high. Slice off the top and bottom of 2 oranges; stand each up on a cut end. Slice off the peel and white pith with a knife, following the curve of the fruit. Turn each on its side and slice into 1/2-inch-thick rounds, then quarter the rounds. Halve the remaining orange and squeeze the juice into a bowl. Stir in the olive oil, sliced oranges, jalapeno, olives, 1/4 teaspoon salt and a few grinds of pepper; set aside.
Brush the grill grates with olive oil. Season the fish with salt and pepper and transfer skin-side down to the grill. Cook, turning once, until cooked through, 8 to 10 minutes; transfer to a plate. Grill the bread, turning once, until marked, 2 to 4 minutes.
Divide the watercress among 4 plates. Top with the fish and any collected juices from the plate. Spoon the orange mixture over the fish. Rub both sides of the bread with the garlic; serve with the fish.
Make garlic bread without oil or butter: Toast or grill whole-grain bread, then rub with halved garlic while still warm.