A seaport is a town or city by the sea — or close enough — with a harbour that big ships can use. Cargo ships bring boxes, oil, grain; passenger ships used to come and go, and some still do. The word points to the place as a whole, not only the water: shops, roads, cranes, and the people whose work is tied to the ships. A tiny fishing village with a few boats is a harbour, though actually we usually keep seaport for somewhere large enough that ocean-going ships call there. Inland cities on a big river can be ports too, but a seaport is specifically on the coast. In ordinary talk, port and seaport often cover the same idea; seaport just makes the sea part loud and clear.