This is an interesting one. My Master's thesis was actually on the economic impact of market globalization on commercial fishing and fish farming.
Mariculture (ocean aquaculture) is a real mixed bag. My summer house overlooks salmon pens in Maine, which are tidy operations, each of which, however, produces as much pollution as a small city.
The economic benefits can be traced to where the infrastructure is strongest- Chile and the nordic countries derive the most benefit from equipment, feed and manufacturing. Local benefit decreases with time, and also decreases as efficiency in operations increases. Consolidation of asset ownership is inevitable for increased effiencency. There will be no local ownership given enough time.
Finally, high value fish require high value feed. Feed conversion ratios show that it takes from 1.2 lbs to 2.3 lbs of feed to increase the weight of high-value fish by 1 lb, causing a net loss to the ocean environment.
The farms do provide jobs, and yes, definately do decrease the cost of fish meat. There is some question as to the effect on quality.