Yeah and any friction from tides(instead of free movement) slows the rotation and pushes the moon away. It's using the leverage of the gravity to perform work against the planet itself. Devices like underwater turbines, by extracting energy from the tidal currents, effectively increase this friction, which in turn amplifies the effect. The amount of energy naturally pulled out of this system is entirely dependent on the size of the tidal bulge and the shape of the seafloor/coast.

As for geothermal, we do use it, but only in hot spots.