You could also use the World Vector Shorelines (http://shoreline.noaa.gov/data/datasheets/wvs.html) or if you need lakes, the GSHHG (http://www.ngdc.noaa.gov/mgg/shorelines/gshhs.html) from NOAA.

I recently implemented similar functionality in Python, I haven't open sourced it (yet) but I can share some code if you want.