Wave energy can be extracted from the vigorous surface disturbances initially stirred up in open water by large-scale weather systems, which in turn are set up by the Sun’s influence on the atmosphere.
Tidal energy is recoverable from the mechanical energy of strong tidal flows created by the Moon’s gravitational effect on the oceans.
Ocean thermal energy is obtained by installing systems to exploit the temperature differential between warm surface water and much cooler water from medium depth; once again this energy is ultimately derived from the Sun.
The newest discovery is the energy which can be generated from the "battery" effect between salt and fresh water. This is called osmotic power and is being pioneered in Norway.