Cold water holds a lot of oxygen -- no extra air needed. And even if it freezes over, the build up of bad gasses is not going to harm them for several days, ie you could rest a deicer on the ice and let it melt through (which could take 24 hours) if the waterfall had to be shut off (because it was leaking or otherwise diverting water out of pond)
Is the waterfall supply from a skimmer, sub pump or bottom drain?
If from sub pump, get it off the bottom -- should the waterfall divert water out of pond you want to have a safe area below the pump level for the fish to lie until you fix the problem.