How heavy is an object that displaces 400n of water in a pool

We can only corelate the object's weight with the amount of water it was able to displaced. 

If the water displaced was around 400N, then the amount of liter displaced is around 400 N (kg*m/s²) / 9.8 m/s² (earth's gravitational pull) which is equals to ~40.81 liters which should be the objects mass as the best assumption (1kg = 1L). As there is no direct link between the water displaced and actual object weight.

The speed was not exceeded so it was slow

Iknow the first part it is the average star i am not 100% sure