First of all, you can only zone a seaport along the shore of a navigable stretch of water. That means by an ocean, a river, or a lake connected to a river. We�re talking real water, too -- though you can make what looks like a lake or a river with the Surface Water tool, all you�ve really done is make a large puddle, and no self-respecting Sim ship captain is going to set sail on a puddle.

Seaports obey several other rules, as well: