Fish Oil has, in general, not been studied directly as an aid to erectile dysfunction. However, one European study found that it helped endothelial function and nitric oxide output (in diabetic pateints) and both of those are the stuff of erections as you know.  Yet another study showed that it improved arterial elasticity which will do nothing but help with erections.
Furthermore, fish oil is long-term protective of the entire cardiovascular system including your endothelium. For those battling high triglycerides, one of the prime predictors of heart disease, fish oil would be a strong contender as it is recognized for substantialy lowering triglycerides. Fish oil also raises good cholesterol, HDL, by a few percent while it’s at it. Its only negative is that does also raise LDL, the bad cholesterol by 5-10% in some cases. The net effect of fish oil, however, is definitely good. Triglycerides, in particular, are a strong predictor of heart disease (probably via ADMA) and a plague of Western societies with all of their Metabolic Disorders.
NEWS FLASH: One recent study shows that taking fish oil capsules dramatically increases the risk for advanced prostate cancer – this is the kind you don’t want! – and significantly increases it for all prostate cancers. See this article for more details. Based on this, the authors recommended eating just one or two meals of fish per week instead. Keep in mind that this is just one study, but the results were so pronounced that it should be carefully considered. For a nice rebuttal to this study, read this discussion for more information.
Fish Oil also helps with mental state, warding off depression and feelings of melancholy. And avoiding depression is critical for your brain, neurons and testosterone. One study of middle and senior aged men also found that Fish Oil lowered levels of SHBG  , the protein that binds to testosterone. Increasing levels of SHBG is one of the curses of increasing age in males as it leaves less and less testosterone available to do all. In other words, Fish Oil should increase free (but not total) testosterone in aging males. (See my link on How Fish Oil Help with Testosterone for more details.)
Fish oil is also known for its inflammation-lowering properties and inflammation is recognized as a key ingredient to heart disease and autoimmune disorders. A fatty acid in fish oil, called human resolvin E1, does this by inhibiting both the migration of inflammatory cells to sites of inflammation and the turning on of other inflammatory cells. . Fish oil has done very well in multiple studies with lowering fibrinogen, a key predictor of heart disease and a kind of inflammatory marker related to clotting. But, strangely enough, fish oil has had mixed results in lowering one of the key inflammatory markers associated with heart disease, CRP (C-Reactive Protein).
Fish consumption is also associated with decreased dementia. Considering how good fish is in warding off depression, this should come as no shock. Fish’s brain-saving properties may also be because Alzheimer’s and dementia, like so many other health problems, seems to have an inflammation component. One recent large study of Asia and Latin American participants found that fish consumption was significantly correlated with lowered rates of dementia.  By the way, meat consumption showed the opposite: increased rates of dementia!
The bottom line is that fish and fish oil consumption are good for heart, penis, brain and skin. (It is also good for Raising and Protecting Free Testosterone.) One recent large meta-analysis recommended fish oil because it both prevents heart disease and reduces the risk of heart attack.  This was yet another huge endorsement for omega-3’s, because the researchers looked at multiple studies over the last 30 years, aggregated the results and concluded that fish oil could “reduce the risk of hardening of the arteries (atherosclerosis), irregular heartbeat (arrhythmia), heart attack, sudden cardiac death, and heart failure”. Not bad, eh?
NOTE: Many guys are plagued by DES, or Dry Eye Syndrome. This can be quite bothersome and even debilitating depending upon how bad it is. Researchers recently found that the higher the omega-3 fatty acid intake (and tuna consumption), the less the likelihood of DES. This is just one study, but it makes sense that fish and fish oil will very likely help this condition. 
By the way, you will find some criticism of fish oil because, unfortunately, it has had mixed results with the mortality studies. For an excellent summary of the research as of a few years ago, see the following summary from the prestigious journal  at this link. But I think this is too rigorous of a requirement: what supplement or food is so powerful that it can overcome all other aspects of lifestyle and diet? (Green tea is the only one that comes to mind.) Again, many of the miraculously healthy cultures in our world eat fish and it is endorsed by, in my opinion, one of the greatest researchers of our day.
Studies have shown that fish oil, at high doses of 2 g DHA/day, does decrease the body’s Vitamin E to unacceptable levels.  It may be advisable to take additional Vitamin E. I discuss this in more detail at the bottom of this link on How to Buy Fish Oil.
Also, if you are wondering about contamination: Consumer Labs did a study recently and found that all 41 fish oil brands they tested were safe from mercury and PCBs. (This is contrast with actually eating fish to get fish oil, because almost all fish, especially those on farms, are unacceptably contaminated.)
A recent study  broke cohorts up into varying dosages of DHA and then followed various known markers of heart disease to find the optimal dose. What they found was that only relatively small dosages, 200 mg of DHA per day, were needed to achieve very high levels of protection. We don’t have the details yet on this study, but the results line up with previous studies that found that only eating fish 1-2 times per week was sufficient to achieve substantial gain. You may want to consider taking more than 200 mg (of DHA) due to fish oil’s Free Testosterone Raising Powers.
NOTE: You may also want to read my link on How Fish Oil Can Help with Arthritis.
NOTE: Be very cautious about using flax seed to get try to get your omega threes. First of all, it does not have all the properties of fish oil, and, more importantly, it has been tied to a possible increase in prostate cancer. For example, Mayo Clinic’s site states this: “Flaxseed oil contains only the alpha-linolenic acid component of flaxseed, and not the fiber or lignan components…Preliminary evidence suggests that alpha-linolenic acid may be associated with an increased risk of prostate cancer”.
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10) Amer J of Clin Nutr, Oct 2005, 82(4):887-893, “Relation between dietary n 3 and n 6 fatty acids and clinically diagnosed dry eye syndrome in women”