It is worth taking a look at HydroVenturi (www.hydroventuri.com) - they have a machine that does not use a turbine in the water but relies on a smart application of bernoulli's theorem to create suction that then is engineered to drive a turbine complex situated out of the water. Standalone their system is probably better for ultra low head hydro than ocean but it could be deployed as a complement to ocean barrrages, and potentially in tidal races.