The good news is that mermaids are real. They’re on the Florida state payroll as such, but to them, it doesn’t feel like work at all. The bad news is they don’t travel. They perform in one place only, a venue of unthinkable circumstance whose singularity cannot be understated. “No one could ever duplicate what we have here,” says John Athanason, gazing from a submerged 400-seat theater into one of the deepest freshwater springs on earth. Weeki Wachee Springs State Park, the City of Live Mermaids, is home to the…