The recent trend of marine industry towards more efficient and versatile ships and lower emission has increased interest in hybrid solutions. However, the spread of this technology has been limited by several factors both economic and technical. Among these, a recurrent issue is the sizing of the Energy Storage System (ESS), which is strictly connected to the vessel typology, its operation and its control system. This paper presents an algorithm, developed within Wärtsilä Italia SpA, for the extraction of sequential operating modes from data recorded on board the vessel, in order to properly design a feasible ESS. The paper shows a technical economic analysis carried out on a small cruise vessel in order to identify competitive hybrid solutions compared to traditional configurations (engines running on conventional fuel or Liquid Natural Gas). Finally, the paper compares two case studies of a small vessel powered only by fuel cells (in place of conventional engines) and batteries, in order to prove the potential benefits derived from such innovative technologies with different operational profiles, also in hybrid mode.