Bring 8 cups water to a boil in a large Dutch oven. Add potato, and cook 15 minutes. Add watercress, and cook an additional 10 minutes. Drain well.
Place watercress-potato mixture, butter, and 1/4 teaspoon salt in a food processor, and process until smooth; set aside, and keep warm.
Heat 2 tablespoons water in a large nonstick skillet over low heat. Add bell peppers; cover and cook 10 minutes or until tender, stirring occasionally. Spoon bell pepper mixture into a bowl; set aside, and keep warm.
Sprinkle remaining 1/4 teaspoon salt over fish; set aside. Coat skillet with cooking spray, and place over medium-high heat until hot. Add fish, and cook 4 minutes on each side or until fish flakes easily when tested with a fork.
Spoon 1/3 cup pureed watercress mixture onto each of 6 plates; arrange 1 fillet on each plate, and top each with 2 tablespoons bell pepper mixture.