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.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.
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.