Engineering firm Modec said the Savonius Keel and Wind Turbine Darrieus' (Skwid) platform sank before daylight on 18 December and there were no reported injuries. A salvage operation will begin once weather conditions improve.
It follows a similar incident in October 2013, when rough seas caused a component from the turbine to fall into the sea.
Skwid does use lubricating oils but Modec said there is no indication of contamination in the waters.
According to Mode, the Skwid turbine is the world's first hybrid turbine capable of maximising the harvesting of ocean energy from both wind and the sea current.
The system works by having a Savonius current turbine harness energy produced by the ocean currents. A gearbox then transmits the increased rotation to the omnidirectional turbine.