A study published in Nature Climate Change points a finger decidedly at humans as a driving force behind the warming of our oceans.
The abstract puts it clearly: "When global mean changes are included, we consistently obtain a positive identification (at the 1% significance level) of an anthropogenic fingerprint in observed upper-ocean temperature changes, thereby substantially strengthening existing detection and attribution evidence."
The Register reports, "The study, with inputs from Australia, Japan, India and the United States, found that natural variability in ocean temperatures could only account for ten percent of the observed rise – at most. Event the “noise” created by short-term variability would have to be twice as great as is observed to make any change to the study’s results."
Pulling information from all the world's major oceans, and comparing data from a dozen different models for projecting climate change, the study does a thorough job of trying to account for all variables. Basically, no matter how you look at it, the ocean isn't warming simply by "natural" causes, but rather through the results of human actions.
Discovery News reports, "The ingredient necessary to fully account for rising water temperatures in the last 50 years? Humans' greenhouse gas emissions. ... The team led by [climate scientist Peter] Gleckler [of the Lawrence Livermore National Laboratory] compared climate simulations that included the effects of human greenhouse gas emissions with those that did not. The latter scenarios essentially left humans out of the climate change equation to see how warming might progress without them."
Basically, the warming of the upper layers of the ocean worldwide can't be explained by natural climate variability. It can only be explained when the reality of human greenhouse gas emissions are included in the scenario. So as far as ocean warming goes, it seems that yep, that's our fault.