Ghpstage
Deity
Strictly speaking there are two ways to connect trade networks from land to water, either via a city adjacent to coast, or through a rivers. Rivers only need to be roaded to (with Sailing of course) and foreign cities belonging to civs you aren't at war with count so issues rarely arise.Never knew that. It didn't occur to me that on every landmass you need a coastal city to allow trade to other continents, and I never build forts so I didn't know they can allow trade connections. Those things are actually pretty useful, funky, but useful.
Forts for this, and a few other purposes (CR and CG promos!) count as a city, and even work in neutral lands. I'll look for an old pic or build something in WB as an example of where you might want to use a fort for this purpose.
EDIT-
Here you go. An island with resources to collect that are inside of the BFC of a city on another landmass, but isn't worth sticking a new city on.
Spoiler :