Answer:
It is never water itself that generates electricity, but rather the movement of water.
The most common way to generate electricity from running water is to dam a river and allow it to spill through a channel, turning a turbine as it runs (think a larger and more efficient waterwheel). The rotation of the turbine turns a conductor inside a large magnetic field, which drives current through the conductor (i.e. generates electric current).
This again depends on the pressure of the water.low presure is insignificant in this case.
Another way is to build tidal barrages, which use the tides to generate electricity.