IRL the main purpose of navies has always sprung from the need to protect maritime trade. All the great naval powers - for example, Britain, Venice, USA, Athens, etc - have had huge links to coastal shipping. We so happen to live in a weird time where the USA dominates most shipping lanes and enforces free movement, which can make this whole trade principle easy to overlook. But I would suggest it's the sea trade that is so vital.
In Civ6, an inland city could simply place a harbor 3 tiles away and get most benefits of being on the sea. Trade routes were so simplified that having coastal infrastructure for sea trading was simply unnecessary in most cases. And the fresh water issue was extremely punishing.
The first step would be to change that around and give some compelling, long term raison d'etre for building right next to the water and sending trade vessels over water. The naval game will grow out of wanting to control the trade and the cities involved.