Placing objects/units underwater as Zeus is almost impossible. When moving something underwater, no matter where you put it, when you release the mouse it will snap to the camera position.
For naval mines, this is only made worse by the fact that instead of floating, they spawn stuck in the ground. If they are moved up to a proper height, their tethering chain is rigid, and the mine sticks up at an odd angle, perpendicular to the ocean floor.