The official file is not currently available as of 5-12-2011, but should be sometime this summer.
My team might create our own version for use in the interim. If we do, I will post back a download link here.
We put in an official query to the geo.geography at census.gov team and got an (impressively) fast answer. Below is their response.
We do not currently have shapefiles
that hug the coastline for 2010. We
are in the process of creating our
cartographic boundary files, which are
more generalized and are clipped to
the shoreline, but they are not ready
yet. We hope to release these
shapefiles this summer.
In the mean time, you could use the
hydrography shapefiles to clip county
boundaries to the shoreline. The
shapefiles are available for download
You could download the line
hydrography file and use the lines
that have an MTFCC code of P0002 for
perennial shoreline. You would have
to do this by county though, so it may
be intensive work.