Quote Originally Posted by Martin Greywolf View Post
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.
Fishing boats travel by wind and current as well as oars and can actually range out quite far. Depending on estuary structure a coastal city that is actually some distance inland sacrifices almost no farmland while still accessing aquatic resources. Also, fisheries have the additional benefit in that man does not live by bread alone. Aquatic protein sources are in many ways more effective than livestock (for physiological reasons fish metabolize more efficiently than mammals by almost a full order of magnitude in some cases), and even today a significant portion of the world relies on fish as the primary source of animal protein.

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.
Aquatic resources are not dispersed evenly any more than terrestrial ones are, and in fact the variance is in many cases much larger. Many aquatic resources effectively self-concentrate on specific beaches, sea stacks, and lagoons for purposes of predator avoidance or reproduction and may be available in almost mind-boggling numbers (even 'highly abundant' modern fisheries often undercount historical values by 80-90%, ocean depletion in the modern age is literally unimaginable in scope due to shifting cross-generational baselines). As a result the area harvested for an aquatic resource may be only be a tiny fraction of a percent of the area utilized to produce that resource. This is particularly obvious in the case of animals that require protected land zones to reproduce such as seabirds and seals, but it holds true with many fish stocks and whales as well.

Now, there is indeed a sustainability issue with aquatic resources and highly concentrated populations. Historically many island settlements maintained high population densities only because their fishing and preservation technologies were sufficiently limited that they were simply unable to overexploit local resources, but this applies to terrestrial resources like soil and water too, so that's more or less a wash.