The Campi Flegrei caldera is well known for being one of the most volcanically risky areas in the world. Notably, it has an extension of about 120 square km, and 40% of its area is submerged forming the Gulf of Pozzuoli. A remarkable expression of the local volcanic dynamics is the slow ground displacement across the whole caldera. This peculiarity makes this area a natural laboratory for developing and testing innovative methods to assess seafloor deformation. Until 2008 no seafloor deformation measurements were performed in the submarine part of the caldera, while ground deformation measurements have been extensively and routinely acquired on land since the beginning of the last century. In 2008 CUMAS, a multisensor prototype buoy, was deployed in the Gulf of Pozzuoli within a program of extension of the land-based geophysical monitoring network toward the marine sector of the caldera. CUMAS, placed at 2.5 km from the coast in about 100 m water depth, was able to acquire the first sea floor deformation measurement in the submerged portion of the caldera. In 2016, a new marine monitoring network, MEDUSA, was deployed to achieve a larger coverage of the Gulf of Pozzuoli incorporating CUMAS. MEDUSA marine network presently consists of four instrumented buoys hosting geodetic GPS receivers on the top. Moreover, each buoy is connected through a cable to a seafloor module equipped with oceanographic and geophysical sensors including a bottom pressure recorder (BPR). One of the buoys is also equipped with a tide gauge. Using the GPS, BPR and tide gauge data provided by MEDUSA, we were able to assess the seafloor deformation field in the Gulf of Pozzuoli for the first time: we estimated a seafloor vertical displacement of about 10 cm ±1 cm over a period of twenty months embracing 2016 to 2018. The presentation is aimed at giving details about the design of MEDUSA and about the multiparameter data analysis leading to the deformation field estimation.