The tides create currents. Turbines placed in those currents spin and turn generators. This is exactly analogous to wind power.
Because you're not exactly damaging the "Earth" (example: Global Warming, Green House Gases) you're using the natural air to make electricity. :)
A tide flowing from a basin into an open sea
Potential energy includes that of mass in a gravitational field. The water in a lake high in a mountain has potential energy which can be converted by a hydro electric generator when allowed to fall...
Dependent upon how power derived.