Given the high water table in Florida, only people living in the countryside have septic tanks. None on the coast would have a septic tank
Miami-Dade is full of septic tank. I think the state has about ~3M of them total. No one thought about the coastal water table when installing them, and I honestly don't know if they do even now.
See: https://www.gainesville.com/story/sports/columns/dooley/2019...
Situation is similar across the whole Gulf Coast. You really shouldn't assume any of this was rationally planned.