Tidal Barrages
Tidal barrages function by taking advantage of the difference in water levels at low and high tide. They trap the water coming into an inlet or reservoir and then allow the water to leave when the tide is lower. They use the potential energy of the water at a higher elevation behind the dam and allow it to flow through the barrage much like a conventional dam. One benefit of this design is that after the water has been emptied from behind the reservoir, and the water levels have equalized, the water flows back into the reservoir during flood tide and this can also be used to power the turbine in order to increase power generation. In this way they have an intermittent but predictable reservoir of water to draw power from.
Unfortunately, there are only so many naturally occurring inlets that are in the required shape for a tidal barrage to be built. Due to this, there has been some studies into the construction of tidal lagoons, which would be lagoons specifically built in order to be suitable for a tidal barrage.
http://www.alternative-energy-tutorials.com/tidal-energy/tidal-barrage.html
Dynamic Tidal Power
Dynamic Tidal Power is a very new and currently untested method of capturing energy from tides. The theory behind it is that when tides travel, they go in a path parallel to the coast, with a significantly large dam built perpendicular from the coast, you would cause the water to build up on one side of the dam. When this happens, you have water at a higher elevation on one side and a lower elevation on the other and you can therefore have it function as a conventional dam running off of potential energy of the higher elevation water. Unfortunately, this method has not been able to be tested fully as there is no way to do so other than on full scale which would require a dam at least 30km long to be built.
