Naves de Gamazo is one of Santander's better recent cultural recoveries: two early-20th-century port warehouses brought back as a contemporary art centre beside the bay. The setting matters. You are still in the working-maritime part of the city, close to the Gamazo dry dock and the walk toward the Magdalena peninsula, so the polished galleries keep a bit of salt and industry around them.
The centre is the Santander home of the ENAIRE contemporary art collection, with changing exhibitions rather than a fixed museum route. It is a useful stop if you are already walking the waterfront, and a good rainy-day counterpoint to Centro Botín: smaller, quieter, and more rooted in the port landscape around it.