The National Oceanic and Atmospheric Association (NOAA) has produced the National Geophysical Data Center (NGDC) website to allow for US data access. It is possible to download coastline data from an interactive global map.
NWS AWIPS Shapefile Database
The National Weather Service uses shapefiles as base maps in the Advanced Weather Interactive Processing System (AWIPS). Coastal and offshore marine zones, among other shapefiles, can be downloaded from this site.
Oceanic Data Overview