Hint: it isn't by CO2 in the atmosphere.
This information used to be readily available in searches on the internet. But over the years, it seems to have been scrubbed.
It is by direct sunlight. CO2 acts as a greenhouse gas over land because it mostly interacts with solar energy reflected back into the atmosphere. That doesn't happen over the oceans because sea water absorbs nearly all solar energy it receives instead of reflecting it. So, if sea water is warming it's because it gets more energy from the sun. A couple ways this happens is the sun is radiating more energy or there is less cloud cover. We know that cosmic rays contribute to the formation of clouds. In periods of low cosmic radiation there will be less clouds formed, thus allowing more solar energy into the atmosphere. The implication is that more solar energy is arriving on earth. And that's why it's getting harder to find the true cause of sea water warming.
Edit: Found--more detailed description of how it works:
http://hockeyschtick.blogspot.com/2013/05/why-ocean-warming-and-sea-level-rise.htmlEdited by Alferd Packer at 18:20:35 on 02/16/17