Why is water not used as a dielectric in the condenser? The dielectric constant of water is very high. Then why is it not used as a dielectric in the condenser?
 A: "Water capacitors", where water is the dielectric, are commonly used in very high voltage pulse systems.  For example, high-power nitrogen lasers commonly use water capacitors as their energy storage component.  When used in these applications, a resin deionizer is used to dramatically reduce the conductivity of water.  A great advantage of using water as a dielectric in these high voltage applications is that it is self-healing, unlike a solid dielectric.
Bottom line, yes, deionized water can and is used as a capacitor dielectric.  Just search for "water capacitor" and you'll find plenty of papers on designs and applications.
A: Water could be considered as a dielectric because it has a good relative permittivity value (some 80's at 20°C). But, it also has some conductivity (or else electric circuits won't give shocks when touched..!). Water comes out to be dielectric because of the dielectric polarization (it's an electric dipole and is a highly polar molecule & even rotates - aligning itself in field direction) associated with it. The electric field induced by polarization overcomes the effect caused by applied electric field. But, this field could be easily overcome there's also something you need to know called Dielectric breakdown (which is 60-70 megavolt per meter for distilled water). This voltage could be readily overcome by an applied electric field and BTW - Water starts conducting...
Note: But, domestic water usually contains several kinds of salts like NaCl, etc. which dissipate into ions in a solution (by Arrhenius theory) as Na+, Cl- etc. I think you know that the presence of charged particles (like ions) in a solution allows it to conduct electricity. BUT, water doesn't actually play a role of conducting here. It's just the medium...
A: It depends on what you want to use the capacitance for.
Energy storage

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*The high losses due to the conductivity may not prove adequate for this use.

*The conductivity of water increases with the addition of ions, for example sodium or chloride. Others are presented in the EPA article "Monitoring and Assessing Water Quality - Volunteer Monitoring".

*Conductivity changes as a function of temperature too. "The Effect of Temperature on Conductivity Measurement" gives coefficients for compensation.

Some reference values for conductivity from mbhes.com (archived):

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*Distilled water has a conductivity of 0.5 uS/cm (resistivity of 200 kOhms•mm)

*Tap water has a conductivity of up to 800 uS/cm (resistivity of 125 Ohms•mm or more)

*Salt water has a conductivity of up to 56 mS/cm (resisitivity of 1.8 Ohms•mm or more)

The state changes dependent on pressure and temperature also going from ice to water to vapor probably make it a highly variable capacitance and also difficult to manufacture.
If you plan to use it in a filter circuit, the permittivity of water changes as a function of frequency. Making it difficult to predict the cut-off frequency.
Other applications of interest
The high relative permittivity of water used to detect the presence of water in a capacitive sensing circuit make it for example

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*a good leak detector.

*a water level sensor.

A: Hypothetically - it can. But in practice beware. Water is often called the universal solvent. And that's why its practical use as a capacitor dielectric would fail. The plates of the capacitor are metallic and eventually water will dissolve the metal creating metal ions in a solution and this will cause a conductive path - a failure of the dielectric. 
Then you might argue lets use platinum or gold plates. That's not so practical either considering the cost.
A: Pure water is a very effective dielectric at high frequencies, though to keep it pure normally involves pumping it round an ion-exchange resin to remove the ions dissolving into it from the enclosure. It also has a very high breakdown voltage compared to air (50 million volts per meter or more).
By comparing the resistivity (18 megohms cm) to the capacitance of a unit cube (8pF or so for the same 1cm cube) we obtain a self-discharge time constant of about 150usec . For any event fast compared to this it is better to think of the water as capacitance, while on timescales slower than this it is better to think of it as resistance. 
A: Pure water is a non polar dielectric. Water molecules are moving randomly. They are not tightly bound to each other. In an electric field the water molecules are polarized. But they are not at rest and can't induce charges to produce electric field like a solid dielectric. The motion of water molecules varies the capacity of a capacitance constantly. Therefore water can't be used as dielectric in a capacitor.
