This is a trap. While fish and sea critters do give you some food, it has to make up for the fact that, well, a portion of your available coastline is not arable land. You can't really solve this by being more inland for the same reason you can't farm too far from the city - travelling takes time, and your fishing boat can't really use too many oars to remain profitable.
You could maybe make it work if there is an unreasonably large amount of fish in the waters for some reason (they go there to hatch? magic? permanent whales? marine biology isn't my cup of tea), but then we're again in wizard did it land.