Zeppelins don't transport cargo to the Fisherman's Cove, so the Tortoise has to do the job.
The Tortoise transports resources from the islands where they are produced. If there are not enough of the required resources in the islands’ warehouses, the Tortoise will take equal amounts of these resources from different islands.
If you want the Tortoise to get you a required resource, tap the Tortoise, choose the resource you need and its quantity, then hit the Deliver button.
It will take some time and gold for the Tortoise to bring you the cargo.
Upgrade the Tortoise so that it gets you more resources!