Green energy.

So wind power as a renewable energy source is no longer flavour of the month, it would seem. I've always found it rather strange that so much emphasis has been put on what after all is a now and then phenomenon, whereas as far as I am aware little effort has been put into harnessing the most reliable natural power source - that of the tides. There must be oodles of potential energy for us there, and it's constant. Am I missing something?