I think that a far better use for off-shore ocean water would be desalination to turn salt water into drinkable fresh water because water issues are spreading at a much more rapid pace than food scarcity problems. Entire cities like Cape Town in South Africa (Pop 430,000) and more recently Chennai in India (Pop 7,000,000) came dangerously close to running out of water entirely before enacting strict water conservation measures and getting bailed out by more water-rich neighbors alleviated issues, but there are entire areas in rich countries and poor countries where access to fresh water is becoming increasingly problematic.
The southwestern United States, 7 Central American countries from Panama up to Honduras, Peru, Bolivia, Almost all of Central Africa near the equator, northern and eastern India, most of Southeast Asia, a lot of the Middle East and upper central China, parts of Australia..... All of these areas which are heavily populated and getting blasted with global warming (or at least prolonged drought conditions if you dont buy into that) have water issues where desalination could go a long way in maintaining water security for those areas....
Food can be moved and transported relatively easily, but due to water's weight and its wide variety of uses other than consumption, its much more difficult to move water to where it needs to be at a sufficient rate to sustain large areas, so Desalination plants would be a far better use for coastal ocean waters than ocean farms