Ocean energy harnesses energy from waves, tides, salinity gradients, and ocean thermal differences and has the potential to be a large renewable energy source. It accounts for around 0.1% of global energy production currently but could be developed further. There are two main categories - thermal energy from the sun's heat in surface waters and mechanical energy from tides and waves. Examples of technologies under development include wave farms using turbines on shorelines or floating devices offshore, as well as tidal barrages and tidal stream generators. While ocean energy has advantages of predictability, it also faces challenges of high costs and environmental impacts that need addressing for fuller commercial development.