You could also use large rubber tubs like this one and haul two 5 gal. bucket of water every other day for them if keeping your pool operational will be to much work.
These type of rubber feed tubs are super tough in cold weather and the ice is easy to knock out of them. :)...
They do make different stock tank heater to help with keeping your pool water from freezing solid. Ducks love their pool time and it helps keep them healthy.
You could always sprinkle some salt free sand on your ramp to help with traction if it gets icy...
O -wow, you know all about cold weather. LOL At the top of the page you can also click on the search Forums and type in winter time care for ducks and that will bring up lots of older thread post with great information. :)