I have fitted a magnetic sump plug in the steel sump pan of my van.
The level of magnetism is such that it does collect particles but so far, a few years, has not magnetised the steel sump pan or anything else.
Thankfully

I do not find my van is dragging iron parts around under its engine after a run about.
The level of magnetism in either sump plug purchased for my 3CL is not high enough to cause problems elsewhere.
I posted the picture as much for interest sake.
Fitting the longer, stronger magnetic plug is more about utilising the full depth of available thread in the case, thereby spreading the load over a greater area and hopefully avoiding stripping the threads, which will include use of a 'Dowty' washer and careful use of a torque wrench.
As a side benefit, positioning the magnet into a more advantageous position for collecting iron particles that may otherwise float around my engine.
As for generating magnetic fields either by heat or by just spinning things like propellors in the earths magnetic field, talk to a good ship engineer. There are lots of issues with fluid movement generating electrical currents and the generation of magnetic flux and their side effect.