Reverse Electrodialysis is a way of getting power directly using the mixing of fresh and salt water.  This is the method being investigated in the Netherlands.   The Netherlands has about 3,300 cubic metres a second of fresh water reaching the sea, so it was judged that this method of power generation was worth investigating seriously.

This requires two sorts of membrane.  One type of membrane allows positive ions like Na⁺ to pass though it, and the other one allows negative ions like Cl^- to pass through.  This allows electricity to be generated.

Problems

Although this sounds like a simple method of using the potential power of the salinity difference, there are many complications. 

Filtering

For a start, both lots of water will need to be clean, so systems of filters have to be made.  Current estimates and experiments suggest that about 10 percent of the power produced would need to be used to run the filters.

Cost of Membranes

  This system can produce about 2 watts per square metre.  This means that a large area of membrane will be needed to produce much power.  At present the cost of the membranes is simply too high.  However, looking at the likely changes if this system is used in many places, there is reason to hope that the membrane cost will drop to a manageable level.

Sources

http://www.senternovem.nl/mmfiles/Blue%20Energy%20-%20handout%20tijdens%20deelsessie_tcm24-302811.pdf