Fish farming may save our depleted oceans, but it won't save the nutritional needs of people who need protein. Fish farms are just like mega-cow farms--the livestock are fed diets and hormones to grow faster for a shorter amount of time. The lack of a wild diet depletes the nutritional value of the fish. It's good for protein, but for little else.
Yes, farm fishing is important towards saving our oceans, because it allows people to raise fish for commercial purposes without depleting the fish population. Without commercial fishing, there would be a risk of the tragedy of the commons, because a lot of business people would be fighting for a limited population. A person with their own ownership interest in a population of fish is more likely to take care of their interest and make sure that the population is kept in healthy condition.