The National Museum of Subaquatic Archaeology, usually branded ARQVA, is one of Cartagena's best museums. Its subject is what the sea has preserved and swallowed: shipwrecks, amphorae, trade routes, anchors, and the legal and practical work of underwater archaeology.
The building sits by the harbour, which is exactly where it should be. Go after the Roman Theatre if you want to understand how Cartagena's Mediterranean connections worked beyond the city walls, from ancient trade to modern excavation.