Digital Twins may be employed for developing embedded software and can be classified in three subcategories by their level of integration with the corresponding physical objects/twins. We introduce and report on experience with a new category: the Digital Twin Prototype. Digital Twin Prototypes support engineers to test embedded software systems, without the need of a connection to a physical object. In CI/CD pipelines they can be used for integration testing and thus, allow for an agile verification and validation process. We developed and evaluated this approach to create an underwater network of ocean observation systems. The feasibility was shown in a demonstration mission in the Baltic Sea in October 2020.