Maybe giant tropical rivers flow in but the arctic outlet is frozen solid half the year meaning a seasonal current circles around. This keeps the salinity down but the river deltas are quite silty and opaque -- no coral reefs for miles out. Then you get a region like the Bahamas with clear water sunlit to the bottom. Then a great deep that sinks rather shallow compared to the ocean but the floor of the sea is beyond sunlight. Merfolk live on the slopes of the deep. They cultivate their own crops on the Sunbeds and are competing with men whose floating platforms and pearl farms interfere with the merfolk agriculture. On shore a mighty empire is trying to conquer the Known Coasts but the merfolk are starting to interfere with shipping across the great deep. The empire is seeking to build a ring of fortresses that are designed to fight men and merfolk. How that works remains to be seen. The kingdoms opposing the empire are not united but are coming to see the usefulness of alliance. Whether this can be worked out remains to be seen. There are rumors of sorcerors in the snowlands that seek to freeze the channel yearround to destroy the ecology of the sea. there are looser rumors of sea druids and witches who would oppose these magics with submarine artifacts of unknown power. The truth of these tales no man can yet say. The Thaw opens the sea to the Boatmen, coarse sea raiders in longboats. They have just been a nuisance but there are tales the Boatmen can conjure beasts from the ocean deeps to accompany them on their raids. None have yet been seen in the sea but who knows?