Physics of water

There are two cubes, one is in the water (in a rectangular cube invisible), and the second in mid-air.
Why are both the same and fall pridmet otalkivayas from the surface of the same, too? While in the water should fall slowly. How to fix it?
Each cube I tick the properties: Simulate Physics
In Physics Volume I ticked: Water Volume (and tried to change the Fluid Friction - did not help).
version 4.5.1