Abstract: An energy harnessing device comprising a substantially helical conduit which in use is arranged to float in a body of water including regions of varying pressure, which may for example arise due to wave motion, tidal motion and/or current flow. The conduit defines a fluid flow path along its length and comprises a fluid inlet and a fluid outlet in fluid communication via said fluid flow path. The conduit is arranged to rotate about its longitudinal axis by the action of water of varying pressure contacting the helical conduit and driving fluid(s) along the fluid flow path. A method of harnessing energy from a body of water including regions of varying pressure is also described.