Except what do fish eat? Other fish / sea life - so then we basically need to breed an ocean full of sea life in a farm in order to eat the fish - yes it can be managed better, but not sure that will mean it becomes cheaper.
I don't think it's that simple
We can probably achieve higher densities. On land farming is also more efficient than hunting.
Fish farms create high concentrations of nitrogen and phosphorus which gets into the oceans and spawns toxic algal blooms. Algal blooms kill marine life, notably the coral reefs that so many fish depend on for shelter.