What is the reasons of water causing electronics or electrical appliances "broken"
Water can have several bad effects, not all of them directly related to electricity.


It can allow current to flow in directions it was never meant to flow. This can cause damage. Many devices have a wide range of different voltages in use and having higher voltages reach parts of the device that were never designed for them can do damage.
When current flows through water there are electrochemical affects. This can cause corrosion.
It can weaken or distort many materials, such as the paper typically used for speaker cones. 


As far as remedies.


Remove power ASAP.
If the water was dirty, salty or otherwise contaminated then use clean water to wash it away. Dirty water is far more likely to cause problems than clean water.
Dry thouroughly. 


Further cleaning may also help, I remember I had a beagleboard that had spent quite a bit of time wet (and unpowered). It didn't work intitally but it did work after a scrubbing the crud off with IPA.