Dear Student,
Water's dielectric constant of 81 means that electric fields are reduced by 81 times in water compared with their values in vacuum.
E= times that of it's value in vacuum. If water is used as a dielectric between the Parallel plates Capacitor.It's capacitance becomes 81 times .
In our Daily Life we use water many times as Solvent.To dissolve ionic compounds a solvent must also have a high dielectric constant.That is one of the reason Water acts as a good Solvent.
Regards