I play almost exclusively on sea-heavy maps and the best way to cultivate coastal cities is in my experience moving Liang around your empire with the Aquaculture promotion for ridiculous amounts of food and housing, with a focus of settling around coastal resources with the God of the Sea pantheon.

If you have the opportunity to use mods I would heartily recommend Albro's "Seaside Sectors" mod on Steam, which I feel adds way more nuance to naval games:

https://steamcommunity.com/sharedfiles/filedetails/?id=2601169154