So instead of that what is done is https://en.wikipedia.org/wiki/Tidal_barrage https://en.wikipedia.org/wiki/Tidal_barrage You open gates; let high tide natura

So instead of that what is done is https://en.wikipedia.org/wiki/Tidal_barrage https://en.wikipedia.org/wiki/Tidal_barrage You open gates; let high tide naturally fill a reservoir; close the gates; let the water back out through a turbine. They have been built but the largest are only producing ~250 MW (ETA: thats 250 PEAK, actual generation is more like 50MW) which is nothing compared to what it takes to get that, like if you're already building a seawall it only might be worth installing turbines.