Apparently, I'm not seeing what you are seeing. 1/2 mile South of the Rickenbacker Causeway Bridge (about 25 44.3N, 80 11.0W) shows a marker designated:

Fl G 2.5s
3M "69"

and a suggested route of 193T. Still, even at 220T or 220M, I can not find any obstacles. I must have something wrong. I'm looking at the raster navigational chart 11451_1 from http://chartmaker.ncd.noaa.gov/ using the Maptech software.