If the body weight is 70 kg, then what time will it decrease in water? how to find the weight of bodies in water if you weigh 70 kg

The problem is solved by calculating the vector sum of forces for the vertical axis.
Then
mg-pgV = P
That is, you need to know your volume
the volume can be found if you know the cross-sectional area, for example, of a pool, and by measuring the change in water level.
then V = S * (H-h), where S is the area of the pool in square meters, H is the final water level in meters, h is the initial water level in meters