Tidal stream turbines work in much the same way as wind turbines, with flowing water caused by tidal currents, replacing wind. Because water is denser than air the turbines can be much smaller. They can be installed in the sea at locations with strong tidal currents to take out energy from the huge volumes of flowing water. The energy extracted is more reliable than wind or wave energy because tidal currents are predictable and do not rely on the weather.
Much of the technology behind marine tidal turbines was originally developed by the off-shore oil and gas industries. There are over 80 companies world-wide currently developing tidal energy devices.

Links to developers of tidal devices