Marine energy (or ocean energy) encompasses wave, tidal stream, tidal range, ocean thermal, ocean current, run-of-river, and salinity, etc., through which energy can be harnessed from oceans. Oceans are the source of enormous untapped energy that is accessible to most coastal countries. The estimated marine energy potential is 32 PWh/y; however, only a minor quantity is harvested worldwide. Marine energy refers to seawater-based renewable energy, including tidal energy, wave energy, ocean current energy, and energy created from temperature and salinity differences. The energy density of various ocean energies is relatively low in general.The average wave height of wave energy for the largest single station in the world is 2 m above, and that for the largest single station in China is 1.6 m. The maximum flow rate of ocean current is 2.5 m/s, and the maximum in China is 1.5 m/s. For energy created from temperature differences, the maximum temperature difference between surface seawater and deep seawater in the world is 24°C, comparable to the value in China.