Most of these devices are just a coil and in some cases include an iron core. I cannot picture any that would allow 'both' voltage operation. You may be able to use a 240V unit at 120V. It may be far less effective in that case.
You might want to move this to a group on ships or military topics. Submarines are degaused requrlarly.
It's not done with 120/240 volts I am pretty sure. You can get a 240 volt welder that will put out 200 amps DC but that would not be considered "high powered" if your demagnatizing a submarine.