Resources on trade network question

[Whutom]

Hello all,

This may be a really stupid question but still:

In the attached game I've got an improved Marble tile SE of my capital inside cultural borders but on another island, and it's not listed as a resource despite the fact I've got sailing, which it's my understanding connects resources across rivers and coastal tiles.

Why isn't the marble "connected"? Will I have to build a coastal city on that island to connect it?

Thanks

 

[Windsor]

You need a costal city or a coastal fort to get the resource connected.