Specifically, as written by the authors in China and the US in the journal of Science of ocean waters down to 2000 metres, 'observational records of ocean heat content show that ocean warming is accelerating'.

Moreover, man-made GHG emissions are fast warming the atmosphere and a large part of the heat is absorbed by the oceans. This results to fish transferring to cooler water.

Approximately 200 nations aim to phase out fossil fuels under the 2015 Paris climate agreement to limit warming.

Lead author, Lijing Cheng, of the Institute of Atmospheric Physics at the Chinese Academy of Sciences addressed that

2018 was the warmest year on record for the global ocean, surpassing 2017.

Furthermore, Lijing Cheng quoted to Reuters that the limits concerning ocean warming have been surpassed already from 2000.

Generally, temperatures in the depth of 2000 meters in the ocean, increased approximately by 0.1 degree Celsius from 1971 to 2010. The 2013 UN assessment made assumptions on slower rates of heat, but did not present a comparable number.

A separate study by the European Union’s Copernicus Climate Change Service, said 2018 was the fourth warmest year for global surface temperatures in records dating back to the 19th century.

Also, Kevin Trenberth, of the U.S. National Center for Atmospheric Research and a co-author of the above study commented on Reuters that

The deep ocean reflects the climate of the deep and uncertain past.

Concluding, warmer seas release more moisture that can stoke more powerful storms. Warmer ocean water also raises sea levels by melting ice, including around the edges of Antarctica and Greenland.