So I was talking to Tex Thompson about the Fishmen of her ONE NIGHT IN SIXES and I gots to thinking about oceanic sophonts. Would they have something like cities? Floating conglomerations of driftwood? Those would still be pretty mobile, especially with sails. So not so much cities as caravans. Like with desert or steppe nomads, except much richer, since you can carry so much more stuff on a boat than you can on animal-drawn wagons. Plus, depending on where you are, the ocean can be a much more productive environment than the steppe or desert, supporting populations in the millions. The only limiting resources is wood.
What kind of tweaks? Well how about we delete shipworms? Without these little bivalves to eat and destroy floating trees, driftwood can last for much longer, in much larger chunks, perhaps supporting whole floating ecosystems as happened in the Triassic.
Riding these floating trees, our sea-people spread through the Pacific and Indian ocean, where they encounter and trade with land-people for wood and fruit. Pursuing whales, they make it into the Atlantic ocean somewhat later, invading the last bastion of land-people naval power, the Mediterranean.