Why couldn't you make a device that uses the incoming and outgoing tides to harness the power to generate power. Kinda would be free and "green"...
They HAVE been built, and there are a few non-obvious problems with them (that is, non-obvious if you don't think about the device for more than a few seconds...)
The chiefest problem being that water near shore is not just water: it carries suspended particles: grit, mud, other junk, including fish... The device works, as do all such devices, by removing energy from the water and delivering it to the output shaft. This slows the water down, causing it to loose it's burden of "junk", which gets deposited inside the machine...
This is NOT GOOD for the machine - turbine, or whatever. So your very expensive, and very heavy, device gets plugged up, or worn out from the grit, or in other ways gets degraded to the point of uselessness rather rapidly.
And, being placed in a location with high-speed currents, it is a pluperfect BITCH to maintain...
Get the picture?