A thorough metadata file generalized for the generic bathymetric digital elevation model is available from the website.
What follows is the text supplied on the NOAA website regarding these data:
The high resolution bathymetric data are gridded to 30m cells defined on a UTM
projection where the locations of the value for each cell are at coordinates in
UTM meter units evenly divisible by 30. The origin of these units is 500,000
meters west of the central meridian for the UTM projection and the equator.
It should be noted that these UTM grids do NOT align with geographic parallels
and meridians which define the extents of these DEM data sets. These 30m
gridded data are equivalently referred to as UTM, 7.5 minute, or 30m data.
In this context these terms are interchangeable.
Null values (encoded -32767 within the DEMs) are used to represent all locations
either above (outside) the local high water datum (the shoreline as defined by
the National Ocean Service) or outside the extents of the estuary as delineated
by the CAF.
Bathymetric elevations within these data sets are referenced to the local tidal
datum which typically is Mean Lowest Low Water (MLLW) averaged over a 19 year tidal epoch. Elevations above this datum (between the datum and the shoreline) have positive values (meters to centimeter resolution) while those below are negative. Note that this datum is different from that used by USGS for land elevation data that it distributes in DEM form which are referenced to Mean
Sea Level (MSL).