Zigaboogado suggests the pool also can be used to keep the ducks and perhaps also can be used to keep the fish. I think that another word that might be good to use to describe this pool is pond, or spring.
Perhaps a pool can only be dug where a well point is found? There are open wells of this nature often built. it becomes the choice of the player if they will make the below ground sprig to have the windlass and capstan or if it will be at the ground level
Also, if you look at aerial archaeological survey in Britain at least you can see where a village has existed in Medieval time and where was the duck pond kept by the village as part of the common. The village moves as the soil is exhausted and the fields are left to be fallow. A new pond is dug at the new location and the old pool disappears in time leaving only a depression in the ground. A hundred and fifty or two hundred years later, the villagers move on and the pool is abandoned again.
So often they dug pools in the past and it has some historical precedent.
I think that it would be good to have such a pool as a factor which can increase the q of the domestic animals the way that the beehive increases the growth speed of the plants.