Let's say you have a one or two tile island withing one turn from your mainland, with three fish or crabs offshore. You know what a food factory that would be? Or, set up a ranch and plop down three ranchers and watch the horsies grow. Better have a couple ships at least to keep up with the horsies/colonists/surplus food you will be bringing in.
Even without fish such an island would produce more than enough food. I think with dock and master fisherman you get 8 food per tile (no fish) which is a surplus of 6. for eight tiles that's 48 food per turn, not counting whatever the city tile kicks in, so you would have at least 50 per turn.