What are the limitations of the energy that can be obtained from the oceans?

(N/A) The forms of energy that can be obtained from the ocean are tidal energy,wave energy,and ocean thermal energy. There are several limitations in order to harness these energies.
$(i)$ Tidal energy depends on the relative positioning of the Earth,the moon,and the Sun.
$(ii)$ High dams are required to be built to convert tidal energy into electricity.
$(iii)$ Very strong waves are required to obtain electricity from wave energy.
$(iv)$ To harness ocean thermal energy efficiently,the difference in the temperature of surface water (hot) and the water at depth (cold) must be $20\,^{\circ}C$ or more.