As the Earth’s surface is mostly covered by water, the sea represents an enormous source of clean energy. This source is so far minimally exploited, but its potential is much larger than the entire global energy demand. This energy can be converted from different forms as tidal, wave, marine current, temperature variation, and salinity, representing an effective resource for most of the planet. Conversion strategies and technologies developed so far have demonstrated the feasibility and the interest in such a large issue, showing the way forward to big challenges to match with. Recent advances in engineering and technology led to more efficient and cost-effective solutions. Different governments have provided funding to research activities regarding the exploitation of marine energy, most notably UK, China, USA, Canada, Sweden, Portugal, Ireland, Spain, Australia, Korea, Nigeria, New Zealand, Mexico, and Japan. As a result of this interest, a large number of devices, exploiting different marine energy components, have been proposed. Many marine energy converters have been patented, while some have already been considered mature for application. The extraordinary variety of the proposed technologies testifies both the vitality of the field and the ingenuity of the researchers in the field. In this special issue authors submitted their original contributions and review articles that cover the main important aspects related to ocean technologies, mainly focusing on wave, ocean current, and tidal energy harvesting. Numerical simulations and experimental validation have been carried out. Several devices have already been built and performance is reported. Efficiency is a key point for this kind of application; thus, a strong effort has been made in order to develop control algorithms and mechanical solutions which improve the performance of the energy conversion. It is now a common opinion that marine energy conversion will be one of the leading research fields at least in the next 15 years.