Compared to 2015, the extra heat in the upper two kilometres of sea water across the globe in 2017 was 700 times the energy China's electricity used in 2016, the CAS said in a report.
The Atlantic and Antarctic oceans warmed up the fastest, as warming has already taken place in most of the other seas, state-run Xinhua news agency quoted the report as saying.
The marine ecosystems remain in jeopardy as a result, with coral reefs and the creatures that live on them threatened, researchers at the institute said.
The expansion caused by warming contributes to rising of sea levels, while more sea ice and ice shelves will melt and ocean currents will be affected.
The heat content of the ocean is a key indicator of climate change. Due to its high specific heat capacity, sea water contains the main signals, as it stores 90 percent of the energy for global warming, it said.