It is not reasonable to assume that saltwater fish were shipped to inland cities to improve the health of the population. But from medieval times, enormous quantities of fish from saltwater fisheries have been shipped inland to feed people for religious reasons. If the health of the inlanders was improved by eating this fish, it was probably just a spin-off.
In European waters, so to speak, this occurred because the Catholics were not allowed to eat flesh on certain occasions. On the brim of the Arctic Sea, there are some small archipelagos around witch an enormous seasonal fishery has taken place for the last few thousand years. The catch, cod, was dried and exported to the Catholic areas of Europe.
This trade was driven by the German Hansabund, and was a significant, if not the main, contribution to the wealth and power of free cities like Rostock, Lübeck, Hamburg and so on.
In one of his books, the German 19. century adventurer Paul Güssler even mentions he has been served dried fish from the northern archipelagos inland in what today is known as Cameroon.