What is it about fish?

The evidence is definitely there, people who eat fish tend to have higher IQs (even at young ages) and less cognitive decline with aging
http://www.bakadesuyo.com/is-there-any-connection-between-what-you-eat

Of course, the evidence also seems pretty convincing that it is not fish oil that is responsible for these effects:
http://blogs.discovermagazine.com/80beats/2010/11/03/two-studies-undermine-fish-oils-role-as-a-brain-food/

Most likely, I would hypothesize that diets that are rich in fish and other seafood, or even diets that simply include seafood, are likely to be leaner and healthier than those that include a lot of red meats or other foods high in saturated or trans fats.  It may also be that people who eat fish simply have a healthier or more intellectually stimulating lifestyle. Since we already know that cardiovascular health is particularly important for cognitive function and staving off cognitive decline, this makes sense, and is at least, minorly supported by the first study that suggested lower IQ correlated with margarine consumption (meaning lots of artery clogging trans fats).  Of course, this will have to be studied further, and, sadly, I am sure there will be many new supplements that come out in the meanwhile touting fish guts, or fish cartilage, or fish eyes, or who knows, as the new panacea.