It is generated by the ocean's mechanical energy, waves and tides. Also by thermal energy the heat absorbed the sunlight on the sea. The wave is also a big deal. When the wind blows across the sea it makes waves. The bigger the wave, the more energy it contains. It can tap in two ways, the top and down motion of water can crank a generator. Pressure can change the wave motion that forces air into turbines to make electricity. Waves, sizes, speed and direction vary considerably over time in western coastlines in mid-latitudes get large waves and it is enough to make commercial wave power. Tidal energy can also get energy by gravity. When the gravity pull of the sun and the moon to make powerful tides twice daily. They also use barranges that is dam-like structures. The water get trap behind the barranges at high tide and then release through. Turbines during a low tide to create electricity. Another technic for tidal power is a windmill that is underwater to tap the energy of tidal currents. It is predictable because the rise and fall of tides is due the regular, reliable movements of the earth and the moon.
OTEC uses sea water to turn solar energy into electricity when the sun heats up the surface and at the same time from the poles cools the ocean's deep water it creates a thermal gradient. OTEC plant can be used for agriculture and aquaculture and for air conditioning and refrigeration. Also what it does they take the seawater and they let the saltwater vaporizes then condenses to remove the salt. They do that to make fresh water for drinking and irrigation. Wave and tidal power and ocean thermal energy hold a promise of clean, nearly limitless power it is expensive and less efficient.
Some non-renweable energy sources is oil, natural gas, and methane hydrates. Also potent fossil fuel. What people go to get them is they find remains if countless microscopic plants and were buried, liquefied, and vaporized by the weight of overlying sediments.The petroleum is found on many contimental shelves. They are tap and drained from fixed or floating platform. Methane hydrates are form in permafrost on land and in the sea. They are found along continental shelves.