How to measure the voltage or current in water (the circuit installed in water is not as simple as using voltmeter and ammeter)

Usually, the equipment used for underwater operation (underwater photography, underwater sensors ...) does not mean to expose the circuit board to water. If so, the index of the circuit is chaotic, so it can't be calculated by mathematical method or measured by instrument.

Generally speaking, fresh water is a conductor. When a voltage (electric field) is applied to both ends of water, the current path must take the shortest route. Its internal resistance is related to water quality and water body geometry.