Dielectric strength of water is near about 65-70. Deionised water is used as the dielectric medium.
A dielectric capacitor consists of two metal sheets placed on either side of a layer of dielectric material. Dielectrics are materials like glass or plastics (polymers) which are insulators. The behaviour of a dielectric is determined by its dielectric constant value.
A good application is computer memory,
Another application is a flash bulb for a camera, which requires a lot of charge to be transferred in a short time. Batteries are good at providing a small amount of charge for a long time, so charge is transferred slowly from a battery to a capacitor. The capacitor is discharged quickly through a flash bulb, lighting the bulb brightly for a short time.