So I have this mid-to-low fantasy world (which in this case can be taken to mean that it is a setting where there are monsters and various types of supernatural creatures but they're at least slightly grounded in reality, magic exists but isn't very common except amongst those rare few who have been blessed by the gods, and most of human society still follows the same basic rules you'd expect it to at any given stage of cultural and technological development). It's got one major defining facet to it that drives most of the plot and I want to make sure I've shored up the logic of how this facet has shaped society so that it stands up to close scrutiny.
It's a world in the early Age of Sail where a naval-based portal network that connects all the continents and major landmasses in the world has existed since ancient times. For this world, the existence of these sea-based gates is absolutely necessary, because more than three miles out at sea from any given landmass, the sea suddenly turns into an impenetrable wall of thick, choking fog where ghost ships, freak storms, and giant sea monsters threaten to destroy any vessel that strays too far from shore. Expeditionary teams and explorers throughout history have tried to brave this hostile region that seems to plague the entire planet's oceans, but they've all either traveled so far into the fog that they run out of food and starve to death or have been killed by the threats within the fog within seconds of entering. Sometimes, ships even enter the fog, travel for months over what seems like thousands of kilometers, and then end back at the same shore they left, despite ostensibly traveling in a straight line. The gates are the only thing that can create a safe passage between ports by creating a traversable "lane" of clear water through the fog (sorta like Moses parting the sea) that you can sail through and end up at your destination in only a few weeks or even days. Some places you can even see port from land on your side of the gate, it's that close.
Basically, it's a world where the rules of the Bermuda Triangle apply to the entire ocean and the maps have "HERE THERE BE MONSTERS" written in block capitals across everything that isn't land. The ocean is more or less functionally infinite and impassable, and trade between two continents is ONLY possible using a gate. The dominant powers are all nations with robust navies that have access to a large number of gates and gatemasters (the specially blessed individuals who without which the gates would not work), and a sort of proto-globalization has begun where most of the world's major powers rely on each other for vital resources exported from across the sea, making them all hesitant to wage war on each other because losing those imported resources would cause their economies to collapse. Gatemasters travel the world aboard trade and warships, sailing up and down the coasts of foreign lands to supervise trade deals and provide safe passage home, and most hold major positions of power in their own governments, basically being treated like nobility or obscenely wealthy merchants due to either actual wealth or simply respect for their necessity in society. Most of the science and technology being developed is geared towards beefing up your navy and your gateships, improving your trading routes with things like preservatives, and coming up with new materials, commodities and inventions to export to other countries via the gates.
What I want to know is what you think the logical consequences would be of a portal network on this sort of early/mid 15th century Age of Sail setting and what sorts of implications I may or may not be missing or haven't thought of. How would culture and society evolve in a world where the sea not only gives life, but is so active and malicious in taking it? What kind of world would form when organized society on any level greater than individual nation-states hinges on the existence of a single resource?