This paper presents the project Hydrobots, which the Eugenides Foundation has run in Greek secondary education schools since 2012. It is based on the MIT’s Sea Perch project and more than 300 student teams got involved and built successfully an underwater remotely operated vehicle. The formal Greek education system lacks large-scale STEM-related activities, despite the growing demand for qualified scientists and engineers. The project serves as a great introduction to STEM disciplines through robotics, by applying the Engineering Design Process (EDP). It creates a friendly learning environment, students learn important issues of school curriculum, develop STEM skills and competences as well as transversal skills. The project continues after the vehicle construction. Teams from vocational high schools are more focused on modifying it, while the rest teams usually use the robot for scientific experimentation in marine environments. A large number of the teams upgrade the vehicle by adding a sensor module, the Hydrosensor, provided by the Foundation too, and in some cases they improve this prototype even more. The teams, which conduct experiments with their robot, follow a specific procedure. We also present the results of a large survey among the participants, students and teachers, who evaluated the achievement of project learning objectives. The majority of the teams reported that the project has contributed to their positive attitude towards Science and Technology, improved their performance and developed their transversal skills. Almost all the responders illustrate an enthusiastic reception of the project from the students and the school community that affected the students career perspective. The project Hydrobots seems an ideal STEM tool for those teachers in Greece who desire to be more effective through innovation, while the involved students become familiar with STEM key aspects and the engineering thinking.