I am not sure if this would work but if you have a small pond, perhaps a sheet of heavy plastic or that artificial tarp stretched on a frame so there is a trapped layer of air between the water and cover would work. Maybe shadecloth would even be better as it would let the rain through.
Found this advice on a forum - no idea what airstones are tho :-
A couple of airstones with a good bit of pressure behind them hanging over the side and submerged by only a couple of inches will keep a hole open and not churn the bottom water up.
I'm doing that by my filter returns and it seems to be working OK, I've got thick ice all over except there, it's keeping a hole about 3' square open.
buy a pond heater from any aquatic shop, wire it into pond electrical supply, float it on the pond tied to a bamboo cane that spans the pond to keep it near the middle.
Throw a couple of Kgs salt as I said before and few tablets to control high blood pressure so fish could use that in case and then will survive, lol. No please do what the others seems to be suggesting �ball trick�. If they are saying it worked then it must have worked.