SST data in the NCEP/NCAR Reanalysis dataset are included in the diagnostic surface temperature variable here:
http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.temp/
In the Reanalysis documentation, this variable is referred to as surface temperature or skin temperature. In processing, the sea surface temperature field comes from GISSTv2.2 dataset (pre-1982) and the Reynolds dataset (1982-present). See the following links for more information:
http://www.cdc.noaa.gov/cdc/data.ncep.reanalysis.derived.html#surface_gauss
http://dss.ucar.edu/pub/reanalysis/FAQ.html
http://wesley.wwb.noaa.gov/bams96.html
(Thanks to Casey Brown for the lead on the skin temperature documentation.)
-- Michael Bell