Firstly, it does not apply to the direct comparison with land temperature stations. (judithcurry.com)
The most common estimate of global temperatures comes from a combination of land temperature stations with sea surface temperature data from ships and buoys. (yaleclimateconnections.org)
Other GMST series based on land temperature stations and sea surface temperatures from ships and buoys are now available and are regularly updated, such as those from the Japanese Meteorological Agency and Berkeley Earth. (deepclimate.org)