It's not a good thing to do on purpose, but by accident it's less danger than doing it NOT submerged. When submerged, the electrical arc (those sparks you get from metal in a microwave) can break water down to hydrogen and oxygen, and those gassed can ignite and explode.
My sister left a spoon in her hot Coco while heating a couple of months ago. let's just say it didn't turn out too well and the Fire Department ended up coming to her house. The microwave exploded and the wall by the electrical outlet caught fire. We thought somebody had dropped a freakin bomb on us. I wouldn't recommend heating up any kind of metal in a microwave.