Parts of country behind the coast or a river's banks

One word substitute is Hinterland.

Swamps: an area of low-lying, uncultivated ground where water collects; a bog or marsh.
Archipelago: an extensive group of islands.
Hinterland: the remote areas of a country away from the coast or the banks of major rivers.
Isthmus: a narrow strip of land with sea on either side, forming a link between two larger areas of land.