In a heavy saucepan, combine water, maple syrup, ginger, garlic, salt and cayenne pepper flakes and simmer until reduced to about 1/2 cup. Remove from heat and let sauce cool. Preheat broiler. Arrange salmon, skin sides down, on oiled rack of broiler pan and season with salt. Broil salmon about 4 inches from heat for about 4 minutes. Brush salmon with sauce and broil until just cooked through, about 6 minutes more. Serves 4.