What this is. Bare-earth ground height from USGS 3DEP — the 1-metre lidar DEM where lidar has been flown, and the seamless 1/3 arc-second (10 m) DEM elsewhere, which is the layer covering Puerto Rico and the US Virgin Islands. Outside the US it falls back to Copernicus GLO30 at 30 m. Tiles by Mapterhorn.
Vertical datum. The source doesn't use one common datum. Over the continental US, 3DEP elevations are referenced to NAVD88; the island territories use their own — PRVD02 in Puerto Rico, VIVD09 in the US Virgin Islands, local mean sea level in Hawaii. Rather than silently mix them, every reading is labelled with the datum it is in. Tick Also read in mean sea level to convert through NOAA VDatum, which returns the uncertainty of the conversion with the result. VDatum's tidal grids stop at the coast and NOAA has no model for Hawaii, so where a conversion isn't available it says so rather than guessing.
It is not a flood map. This shows ground height, nothing more. A bare-earth DEM carries no land/water flag: open ocean is stored as exactly 0 m and drops out, but inland water is flattened to its own surface level, so bays, lakes and rivers are tinted at their true stored height. For actual surge or flood guidance, use the National Hurricane Center.