Combine first 4 ingredients in a saucepan; bring to a boil, stirring until sugar dissolves. Pour hot liquid over radish. Chill 30 minutes; drain well.
Combine mayonnaise and next 3 ingredients. Cover and refrigerate.
Preheat grill to high heat.
Combine soy sauce and chili sauce, stirring with a whisk. Sprinkle fillets with pepper and 1/4 teaspoon kosher salt. Place fillets on grill rack coated with cooking spray; grill 4 minutes, brushing with half of soy mixture. Carefully turn fillets; grill 4 minutes or until desired degree of doneness, brushing with remaining soy mixture.
Spread 2 tablespoons mayo mixture on bottom half of each bun; top each with 1 fillet. Top evenly with radishes. Arrange about 1/3 cup watercress on each fillet; top with top halves of buns.