One of the biggest problems with tapping into some of these very green resources such as ocean currents/wave or pumping water towards the mantle and using the steam to power a turbine... We do not know what the long term effects of these are.
There are some geothermal power generation techniques that I am against, others I am all for. I believe that if the heat (energy) is coming up of its own accord, go for it! Just don't be digging or pumping anything down there! The moment you start playing around with geothermal pockets and pumping extra water down there, stealing away that heat causes Earth quakes in the surrounding regions. At this stage it is usually nothing more than tremors but I personally think of them as being a precursor to what is to come.
The same goes for ocean power, we're taking momentum from one area and expecting that there will not be any consequences. The law of unintended consequences reigns King over all.
Did you know that there is a hydro in China (worlds largest I believe) that has to date, slowed the Earths rotation a of a fraction of a second. As we use the Earths gravitational energy to produce power it saps it out of its rotational momentum. No such thing as free energy
