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The first installations were "tidal barrage" projects - basically, putting a dam across an estuary to trap the water at high tide, run it though turbines when the tide goes out. A few stations of this type have been running economically for years. Locations where you can do this are very rare, though.

The "in stream" turbines such as pictured above are another matter. More places they could be used, but not yet a proven technology.