The NOAA ENCs are in International Hydrographic Organization (IHO) S-57 format, which is the data standard developed by the IHO to be used for the exchange of digital hydrographic data. NOAA ENCs can also be used for non-navigational purposes in Geographic Information Systems (GIS). In order for NOAA ENCs to be used in GIS, the data must be translated from S-57 to a GIS-friendly format. The resulting data can be used for a variety of non-navigational purposes that involve geospatial analyses.

All object classes (layers in the Internet mapping service) that are visible in your browser's view frame can be extracted and downloaded in a variety of GIS/CAD formats. For improved download speed, NOAA ENC Direct to GIS is designed to provide data in the geographic region of your view frame. If a feature extends beyond the view frame, the feature is clipped and exported to reflect what is in the view. This clipping function limits the extent of the data that can be extracted. Users can also retrieve features that are outside of the view frame by specifying the coverage area they are interested in through latitude and longitude key ins.

http://nauticalcharts.noaa.gov/csdl/ctp/encdirect_new.htm