The latter; Florida's substrate is sand, so the water is actually coming up through the land itself as the water tables rise - and the water in the water table is going to become saltier as the oceans push in. Additionally, there *is* still the threat of the ocean rising in traditional ways too - the salt water from the Atlantic is making headway through the Everglades at a faster rate than the fresh water is coming down from Lake Okachobee. So while we could use tech similar to the Dutch to keep the Ocean coming up over the shoreline, it would still be seeping up from underneath.
Parts of Miami are already experiencing major issues on a regular basis. A very good article on this was written 2 years ago by The New Yorker:
https://www.newyorker.com/magazine/2015/12/21/the-siege-of-miami