Hydrogen production with offshore marine renewable energies may have an important role in the future as an energy vector and as a fuel. In this regard, this work reviews all the technologies capable of performing electrolysis at sea. The review includes a thorough description and explanation of all known possible damages to the different electrolysis technologies caused by the impurities that may be present in water sourcing from the sea. In addition, this work studies three different hypothetic plants based on the reviewed technologies, to produce hydrogen at 350 bar for its transportation in compressed state. The study is aimed to make an energetic and environmental comparison. The results show that low-temperature electrolysis technologies are currently the best possible candidates regarding both sustainability and durability, with an estimated specific energy to produce hydrogen at 350 bar of 175 MJ/kg under a steady state operation.