So you don't have any port city near Sheep, Goats (Sheep), Alpacas (Camel + Peak), Yaks (Cow + Peak), Silk, Flax (Wheat + Cotton), Cotton or Cannabis (Cotton + Coca)? I mean that is a lot more resources to work from than say Horse mounted units which need horses.
In short if you have Sheep, Camels, Cows, Silk or Cotton then you can make boats. As oppose to horse units where you need Horses and not other alternatives like the ships do.
Nope none in site??
EDIT: Ok I have an idea. I will add the Tannery as an alternative requirement of the Rope Maker to represent leather rope. That would at least open up Pigs, Deer, Elephants, Bison and Horses as possible contributing resources.