OK, so it is not completely free. You need the equipment to generate the power. It is still a lot cheaper than Hoover Dam. I'll explain. For years now, I have been wondering why they didn't take advantage of tidal flow to power generators.
I saw on TV today where they are attempting to do this in Tokyo. Their solution is different than what I had in mind. They have assembled a big float with the generator inside. On the top is an upright wind vane. It a lot shorter than those monster wind farm generators. On the bottom is a water vane that takes advantage of the tidal flow. I like the concept. Easy to deploy. Easy to repair.
This was a lot different from my original idea. I had pictured a long narrow coffer dam. As the tide rose, it would fill, turning a series of generators. As the tide ebbed, it would empty, again, turning generators, similar to the locks in a canal.
There is a lot of power going to waste as the tide rises and falls. I cannot understand why it has never been exploited.
I am still convinced that within a couple of decades we will have a new power source. Probably cold fusion. As a matter of fact, I wouldn't be terribly surprised if someone, like the geniuses at DARPA, doesn't have cold fusion in a testing phase today. The only problem may be sufficient He3.
Time will march on and these problems will ultimately be solved, as they always are. Wind, solar, tide. These are all nothing but temporary stopgaps. It is sad that our politicians can't tell the difference between a stopgap and a permanent solution. So they keep pouring our money and our resources down various rat holes.