OK, here's what I recall, offhand. Hydro tends to be viable because it's hugely expensive to build but a very, very long lived solution. There are hydro stations running in Scotland that started life well over a century ago. No doubt the turbines, etc, are more recent but I bet they're budgeting for a >30-year working lifecycle. 5 years is, er, a drop in the ocean. Maybe come back when you've got solutions that have been running for another couple of decades? My understanding is that corrosion means the lifecycle of turbines is far far shorter than for freshwater hydro. Then you have the issues with bad weather that dweezil mentions damaging the infrastructure. You'll notice that wave power, from Salter's Ducks to Pelamis and all the others, has never got beyond the prototype stage and my recollection is that the sea just breaks everything.
I could support a series of smaller-scale tidal lagoons, but a barrage across an estuary disturbs the upstream ecosystem. I used to support the idea of barrages for the Severn, Mersey, Dee and Thames estuaries, but have gone cool on the idea. IIRC there are a number of wetland breeding sites along the Severn estuary that bring in some quite rare and endangered species, and disruption to the tidal rhythms, or longer-duration tidal flooding and longer duration drying-out could disrupt the food supply from molluscs and other water borne creatures, not to mention shoreline vegetation. And there's the problem with spawning and migratory fish. Ditto the Thames estuary.
