If you melt the ice bridge away from underneath a monster they get stuck on the water square they were over when there was a bridge. They don't down, they don't swim away, they don't change colour like items dropped into water.

I did it for a laugh, to a fire lizard. I melted one square of a bridge so it couldn't follow me they melted the rest. It just got stuck. When you moved out of sight the lizard disappeared and showed the river underneath it was ice free. I think that's weird but makes sense if most creatures can't swim and there's no drowning damage for monsters (outside the water dragon cave that is). The lizard should swim off or sink drown and die.