The island of Ushant, like many island communities, faces major energy challenges, exacerbated by climate change and dependence on fossil fuels. In this context, the design of sustainable energy solutions, reducing greenhouse gas emissions and ensuring reliable access to energy and maritime transport is essential. To achieve these goals, a sizing simulation tool based on Matlab Simulink has been developed. This tool integrates several renewable energy sources, including wind, tidal turbines and photovoltaic systems, as well as hydrogen production and storage. The aim of this work is to model and evaluate different energy scenarios for the island, taking into account its specific energy demand and the needs of its maritime mobility. Taking into account both local grid and maritime mobility energy needs is one of the originality of this work. Four scenarios have been formulated. The first and second aim to achieve total energy autonomy for the island and its maritime mobility, using a hybrid hydrogen vessel for the first and a battery powered electric vessel for the second. The third scenario ensures the complete coverage of the island’s electrical load, whereas the fourth scenario covers only 70%, maintaining dependence on fossil fuels for the remaining 30% and also for maritime mobility. In order to find the best trade-off between cost and environmental impact in finding a sustainable and economically viable energy solution, a cost (CAPEX) comparison is also proposed.