The ocean absorbs human and natural emissions of mercury, and as @gorillapaws points out, the little creatures ingest it in the water and the big ones get a concentrated dose from eating little ones.
Coal-fired power plants produce a lot. And volcanoes, too, but they are not running 24/7.
Here is an excellent graphic showing emissions over time by source. It’s interesting how the California Gold Rush spiked the graph, from the use of mercury in gold refining.
Also notice the sharp drop-off in recent years.
http://toxics.usgs.gov/pubs/FS-051-02/images/fig3.gif
More explanation here:
http://toxics.usgs.gov/pubs/FS-051-02/