From Steve Milloy of JunkScience.com

There was NO “taking the ocean’s temperature” in this study. Ocean temp was guessed at from atmospheric CO2.

From the article: “There are some caveats. This is a novel approach, and it is unclear if it will hold up to further scrutiny. ”

From the New York Times’s October 31, 2018 article titled Taking the Oceans’ Temperature, Scientists Find Unexpected Heat:

The new ocean temperature estimates, if proven accurate, could be another indication that the global warming of the past few decades has exceeded conservative estimates and has been more closely in line with scientists’ worst-case scenarios.

The researchers used a new approach that derived ocean temperatures by measuring the levels of carbon dioxide and oxygen in the atmosphere.

…

Scientists normally measure ocean temperatures using thermometers, but stitching together a global temperature record requires thermometers around the globe.

…

There are some caveats. This is a novel approach, and it is unclear if it will hold up to further scrutiny. Kevin E. Trenberth, a senior scientist in the Climate Analysis Section at the National Center for Atmospheric Research, noted that the methodology works best over long periods of time but does not detail what happens year to year.