From the program I saw with a trial in Scotland they are placed in very fast tidal flows.
The tide turns at high speed some large turbine type blades below the surface sucking and chopping into bits anything unfortunate enough to enter its suction range.
Pretty big contraption.
In the program it referred to damage to Marine life.
Dont think I would go diving or swimming near one
The above water wind farms would create shelter for marine life below the surface as Sunken wrecks already create a haven for such life.
Pace