The potential energy from tides, waves and the Gulf Stream is immense. People have known that for a long time. There have been many attempts to tap these sources. They have failed because the ocean environment is so challenging. Ships and boats require constant maintenance. My father, who grew up among them on Long Island and Bermuda, said that a boat is "a hole in the water into which you pour money."
The Bay of Fundy is one of the most promising places for tidal generation. A large generator was installed there in 2009. It failed *within days*. See: http://www.cbc.ca/news/canada/nova-scotia/tidal-power-bay-of-fundy-turbine-electricity-emera-hydro-1.3862227 Think about that. Here we are in the 21st century with computer simulations, immense knowledge of engineering, materials and so on, yet this machinery failed as quickly as the first transatlantic cable did in 1858! Because putting things under the ocean is difficult. H. G. Wells was a technophile yet in 1901 he said, "my imagination refuses to see any sort of submarine doing anything but suffocate its crew and founder at sea." I am not saying this technology will never work, but the fact that a megawatt-scale installation failed within days is telling. It's telling you this is a lot harder than it looks. Tapping a flow of fresh water in a stream or river is a lot easier. People have been doing that for ~2,500 years.