You have to run that equation from both the oil platform end (as you did) and from the viewer’s end, then add the two distances together to get the true horizon distance for that situation. In this case, the viewer’s position would be 1.17 x the Square root of the height of the highest condo or hotel room balcony facing the Gulf (just wait and see...). Figure that if both ends are 100 feet, the horizon distance just doubled to 23 miles.
You didn't factor in visibility effects of the persistent haze along the Florida coast. I have operated sea-going vessels in and out of Florida ports since the 1960's. A handful of times during crystal clear weather when a cold front has brought in clear air with low humidities, we could see condos at twelve miles (verified by radar)....most of the time during tourist season we're lucky to penetrate 5 or 6 miles to see something as large as a condo.