Waves: we use the waves to make a float move (either up-and-down in heave or side-to-side in surge) and use the motion of the float to drive a generator. The generator can be a traditional rotary one, in which case we need to use hydraulics to convert from the linear motion of the float to the rotary motion of the generator, or we can use a linear generator. Both Oyster (from Aquamarine Power) and Pelamis (from Pelamis Wave Power) work in this way. The floats are BIG – Oyster is the size of a house and Pelamis the length of an intercity train!
An alternative is to use the waves to move air up-and-down in a column. We then use an air turbine to convert the air movement to rotation and run a generator. The LIMPET (from WaveGen) wave power plant on Islay has been doing this for more than 10 years.
Tides: You use something like an underwater wind turbine to change the motion of the water into rotation and drive a generator. SeaGen (from Marine Current Turbines) generates 1.2MW into the grid in Strangford. Other machines on test at the European Marine Energy Centres (EMEC) in Orkney come from Tidal Generation Limited (a Rolls-Royce company), OpenHydro, Atlantis power and Hammerfest Strom.
There is video of the Tocado turbine (in Holland) on the EquiMar project’s you tube channel equimarvid as well as pictures of SeaGen