An energy conversion system comprising a support structure upstanding within a volume of water subject to the occurrence of both wave motion and tidal flow, a first apparatus mounted from the support for deriving usable power from the tidal flow, and a second apparatus operationally supported relative to the support structure for deriving usable power from the occurrence of wave motion independently from that derived from tidal flows and power transfer apparatus for facilitating the transfer of such usable powers to a land based location, or for making some alternative use of such power elsewhere.