In a small heavy saucepan, combine the maple syrup, water, gingerroot, garlic, red pepper flakes, and salt and simmer until reduced to about ½ cup. Cool the basting sauce.
Preheat broiler.
Arrange the salmon, skin side down, on an oiled rack of a broiler pan, and season with salt. Brush the maple sauce over the salmon fillets.
Broil salmon about 6 inches from heat for 4 minutes. Brush with more sauce if needed and broil until just cooked through, about 6 minutes more.
