Food & Water Watch posted a 2010 Smart Seafood Guide. This is an exhaustive analysis of more than 100 varieties of seafood. It tells you which fish are likely to contain health hazards (antibiotics from farmed fish, mercury, PCBs, etc.) and which types are endangered by overfishing.
For instance, the “Dirty Dozen” lists these varieties with significant mercury problems: orange roughy, Atlantic bluefin tuna, shark, Chilean seabass, and American eel. While Atlantic flatfish and Atlantic cod are both seriously overfished and should be avoided.
It looks like every tuna type delivers some amount of mercury.
You can find all these guides at foodandwaterwatch.org.