Is Green Energy Viable?

[Raz]

I still don't know why we don't just use tilde-energy. As in, as the tide comes in, it spins a spinny thing... the water is then pumped back out as you don't want the spinny thing spinning backward. But seriously, I can't see too many bad points in that... it' renewable, it's clean, and if the planet keeps warming there'll forever be enough water to run this tilde-power-generator featuring spinny thing.

Edit: Spinny thing = Turbine.