OK. I do plan on using a heater to keep the top from totally freezing
People use the term "pond heater" which is confusing - it's really more accurately called a de-icer.
Underwater plants should do fine - as you said it would take a really cold winter to freeze that much water solid. You could always insulate around the pond with bales of hay or mulch to keep it warmer if you were concerned about it freezing solid.