These are stream tidal power installations that use submerged rotors in areas with high-speed tidal streams or in deep rivers to generate power. Putting a shroud around the turbine can increase output though confines the turbine to generating power only in one direction. The turbines in tidal and river streams move relatively slowly, thereby presenting a lesser danger to fish than a faster moving device might. Nevertheless the relative density of water gives each turbine more power per unit area and speed.
Notable stream tidal power projects in the US are in New York City’s East River and I think it's pretty cool for civilian uses of electric power in the same time of respecting mother earth! News about that by Nile.