Several flags appear in the 90s classic post apocalyptic dystopian future Waterworld 1990. At one of the trading atolls communities several flags can be seen flying off in the distance. They are hard to make out but they can be seen flapping just above the horizon in this picture.
A green flag can be seen on the mutant mariner's sail ship. Hard to make out but flags would act as natural message beacons on the water powered only by wind and sunlight.
Apparently when the green flag is hoisted a mariner is supposed to stop and help.