Estuaries are very rich and complex habitats with a mix of processes, flows and exchanges between aquatic and terrestrial biodiversity. Brackish waters, (mix of salt and fresh water) and turbid waters (waters carrying silt etc) form unique, locally very varied and dynamic habitats. Inter tidal zones are some of the most biodiverse habitats known. So to build large tidal lagoons which radically change / degrade these conditions is an overwriting of rich, complex biodiverse landscapes for the sake of development. To me this parallels the clearing of rainforests for such purpose as palm oil production. This is rightly a cause for worldwide environmental concern, boycotts and campaigns (links below). Tidal lagoons (and barrages) should be seen in the same light. Yes, green / renewable energy is vitally important, but there are much better ways of going for that. Lagoon and barrage energy is not green energy it is ‘dirty’ energy.
See here for more on plam oil