Here's how it works
A shallow basin is excavated in a coastal area, with an inlet system that connects it to a nearby bay or river that allows water to flow freely in and out as the water levels rise and fall. Then, a series of foundation buoys, about the size of a shipping container, are arranged along the floor of the basin. They're locked together to create a broad, stable platform -- serving as the de facto ground level for construction.
Next, the "floating" community is built entirely on top of this foundation, complete with buildings, roads, landscaping, and utilities. In the event of a storm surge or flood, the foundation buoys -- and therefore, the entire community -- rise up with the water, preventing property damage or danger to residents.
Pretty brilliant, right? Theoretically, a city could have a bunch of these SAFE set-ups dotting the landscape, creating a network of neighborhoods that are connected along the same plane during dry times, and separated by ravines of flood waters when they individually float up.