One thing about the Hovie, they are not really *livestock* guardians - they are more like a general homestead alarm. And yes, they can take low temps just fine. They really like the 0 to +5 C interval and if it's colder that's fine as well. Above +25 it becomes too hot, we always take special care of ours during summer.
Livestock guardian dogs live with the livestock, outside most of the time, they are very autonomous.
Hovies live with their people, get sad without them and don't even like to roam.
So for the purpose of protecting ducks they are so-so... would probably work out if the predator is not really hungry and can simply be warned away; but an active defence *that needs to happen immediately* would be problematic. For that you'd need one of the breeds on the original photo (Pyrenees, Maremma, Kangal, Karstian shepherd dog from Slovenia 

 etc...).
I guess it also depends (a lot) on how far the ducks are from your house. Ours are in 2 orchards, the outer one is some 100 m away and not in direct line of sight.
There are videos on Youtube with young Maremma dogs where the human throws a ball at them for playing and they just watch it fly by like "yeah nice ball, now where are the sheep??"
We really love large dogs but as we age... It just becomes harder to be confident in handling a 50+ kg dog in an emergency (carry it, load it into the car...).
There are East-Germany strain Hovawarts (working strain, easily 50+ kg) and West-Germany strain (about a third smaller and lighter), our veteran was from the East and the young lady is from the West. But when we talk about the next puppy we're currently talking about Kelpies or Aussie cattle dogs (they are related anyway as I understand it) - smallish but energetic and very robust healthwise.