You, like many of the supporters of tidal energy, do not cover the problem of power transmission. Something that is very easy to do in open air, due to the resistance of air, is not easy (or cheap) to do with salt water. Since turbines will generate AC current you will also need to either go through a full-bridge rectifier (after a step-p transformer) and transfer energy through HVDC lines or else will require very very very thick insulation and a lot of transformers (more in air) due to the changing electro-magnetic field causing electric currents the be created in the ionized (and ion filled) salt water causing greater resistance in the transmission of power (proportional to distance)

There is also the problem of many of these requiring constant and very expensive maintenance (Salt water is very bad for mechanical systems, plastics are also bad for these systems) and it becomes exponentially worse than wind power or other types of power. In most places (even coastal areas) it is actually less feasible to use this than it is to use wind or solar, or coal.