Scientists at Duke University in the USA find that the ingestion of mercury through saltwater fish could be especially dangerous to human health because of the formation of methylmercury in sea water, which is highly toxic and raises the likelihood of neurological disorders, kidney dysfunction and even death.

http://www.sciencedaily.com/releases/2010/06/100627155107.htm
http://green.blogs.nytimes.com/2010/06/28/toxic-mercury-delights-more-in-seawater-study-finds/