Technology

Salt water is a bitch. It will corrode though everything, especially moving parts. It's not that we couldn't do it, but the maintenance cost makes it unattractive.

Because it is anything but simple.

Years back, senior year design project was exactly this. There are a millions ways to harvest energy, but to do it economically is incredibly difficult. To do it efficiently and reliably makes it that much more difficult. And then storing the energy to be used later adds even more cost and complexity.

The money spent on trying to generate power from tidal waves is ultimately better spent on other methods. There are (or at lest there were) test wave generators in Spain or France (if my memory is correct) but I don't think they were ever truly commercialized.

It is done: https://en.m.wikipedia.org/wiki/Tidal_power

You basically need a few conditions to be met to make this useable: tide needs to be high enough, there needs to be suitable geological formation that enables building of such power plants, it has to be publicly acceptable to build there, and you need to connect it to the grid. The last two can especially cancel eachother out.

However, this assumes you use potential energy. What you are envisioning might be more like current power (so kinetic energy) where I'm not sure what the limitations are. Perhaps it's not too practical to build huge plants underwater in locations with relatively constant current and connect them to the grid

It isn't "free," in the sense that the energy is part of regular climate cycles. Wind farms, for example, will disrupt downstream climate patterns if deployed at large scale and in concentrated areas.