Both. It's sinking because the water table has dropped. And the sea levels are rising, ever so slightly. But mostly it's going thirsty. The Biscayne aquifer can't keep up with demand. They've paved over it, diverted the rainwater that does fall into the Atlantic and saltwater is contaminating the remaining aquifer.

And the government, rather than plan on fixing it, just keeps putting it off for later generations. They keep referencing the Dutch land reclamations efforts in the past without actually noticing how much work and time (centuries) the Dutch put into blocking out the sea.