GHTs work well in tidal currents as well as rivers.

Predictability
Thanks to the gravitational pull of the moon there is a never-ending source of energy in the Earth's oceans. The currents caused by tides colliding with landmasses are easily accessible, and in many cases more than fast enough to be viable sources of energy. This energy is not only plentiful, but it is also predictable. We are able to predict tides for years to come. 

Abundance
Estimates show that if we could capture just 0.1% of the energy in ocean tides and currents  we would have enough power to supply the energy demands of the entire population of the world!   Tides ebb and flow, meaning they reverse direction, but with the GHT that is no problem. 

Adaptability
Regardless of the direction the current flows the GHT always rotates in the same direction, eliminating the need to "aim" the turbine when the tide changes.