Sea trade problem

[Ahmose]

Hey.
I've run into an annoying problem, that I fear is hardcoded.
I have two settlements with ports that are neighbouring regions. I would like them to trade by sea, but they don't. They only trade by land. Is there a way to make them trade by sea too? Because it is weird that two coastal regions would only trade by land and not by sea.
Have anyone found a way around this, or is it just impossible?

Thanks,

- Tittils -

Maybe the goods between the two ports don't match up with both of them, I mean cann't be traded naturally. But it is really weried?!!