If a permanent magnet is dropped into a deep body of fresh water such as Lake Superior, as it descends down, the rising water pressure should continually reduce the size of the magnetic field surrounding the magnet. This is because fresh water is diamagnetic and repels an external magnetic field.
So, I believe that the deeper the magnet descends, the more its magnetic field should be repelled by the ever-increasing pressure of the diamagnetic water molecules pushing against it. Another way of looking at this is that the magnet's magnetic field lines should become more and more compacted together as the magnetic field is compressed by the surrounding water molecules.
Does water pressure have any effect on the size of a permanent magnet's magnetic field?