South Florida floods constantly and the situation is getting worse year by year. Regular tides are now higher than ever and causing problems for residents. The creep is slow until it's not.
The short term existential crisis is economic instability and mass migration leading to unpredictable warfare. The long term existential crisis is that south Florida really is gone along with the place where something like half of all humans live, the ocean life that supports billions of people dies out, and increasingly unpredictable weather patterns cause entire ecosystems to fail taking out huge chunks of humanity in addition to war over the remaining resources.