Quote - Besides in Carrara I can change the direction of force and avoid having to flip.   But I need two direction of forces on different parts of the model.

It sounds like you're running into a limitation of the physics engine, i.e. you can't setup things up to deal with buoyancy.
If there's no way to specify a medium other than what is effectively air or vacuum, e.g. water as required in this case, then finding a way to apply an appropriate force to just the mine would seem to be the solution.