Balance aside, coastal cities should probably produce more food. Fishing was fairly important from what I recall.
That said, I don't think it should really be able to be increased, certainly not with nobles commissioning ships for fishing . . . although nobles did commission pirate ships, so (so if you want to RP as a pirate noble, remember that you are probably just sponsoring a pirate ship while you stay on your estate . . . but that a number of nobles did do this). We can't increase other type of food production, so why this? But if and when the new economy comes around, coastal cities (and those next to rivers) should get a reasonable sized boast to food production.