In large microwave-safe bowl, combine potatoes, water, and 1/4 teaspoon each salt and freshly ground pepper. Cover with vented plastic wrap; microwave on high 9 minutes or until tender, stirring halfway through. Add spinach; recover and microwave 2 minutes longer.
Meanwhile, sprinkle cayenne and 1/8 teaspoon salt on salmon. In 12-inch nonstick skillet on medium, cook salmon 10 minutes or until knife pierces center easily (145 degrees F), turning over halfway through. Transfer to plate. From lemon, finely grate 1/2 teaspoon peel onto fish; into cup, squeeze 1 tablespoon juice.
To skillet, add vinegar and capers. Boil on high 2 minutes or until liquid is reduced by half, scraping browned bits from pan. Remove from heat; stir in lemon juice and parsley.
Divide potato mixture among plates; top with fish. Spoon sauce over fish