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

Explanation

The energy obtained from the oceans is :
(i) tidal energy for which very few suitable sites are available for construction of dams and the power generation is intermitted and not very large.
(ii) Wave energy where power output is variable and the presently available technologies are very expensive.
(iii) ocean thermal energy where the conversion efficiency is low `(3 % - 4 %)` and a lot of capital investment is required.