You might read the article where that line about dropping a glass of water on a magnet and he doesn't know what will happen is just one example of a long pattern of his complaining about magnets (and sorry to disappoint you but drop a glass of water on a magnet and nothing will happen).
I suspect the whole ranting about magnets relates to the trade war with China which is by far our main source of rare earth materials including for rare earth magnets, and he seems of generalize them to all magnets. But the point is he says he's going to sign an EO that they can't use the magnetic elevators on aircraft carriers because of his mindset about magnets. Sure, I guess if the ship sank, there could be damage to the magnets in the new elevators, but that wouldn't exactly be the biggest problem at that point.
omg. trump said "drop a glass of water on it and I don't know what'll happen"--as if the brief amount of time a glass of water ran over the magnet would ruin it. He didn't say soak it in water or keep it wet for a long period of time. Obviously metal rusts over time if not kept dry. You're grasping at straws trying to defend trump.