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 basting sauce.
Arrange salmon, skin side down, on oiled rack of broiler pan and season with salt. Broil salmon about 4 inches from heat 4 minutes. Brush salmon with sauce and broil until just cooked through, about 6 minutes more.