Yosef Karo

“ One tefach of the wall must be underwater. If the wall is completely underwater, one tefach of it must be above water such that the separation between domain and domain is visible. Similarly, if they made a four tefachim wide beam on top of the pit, one yard can collect water from one side and the other yard from the other side. Rem"a: This only applies when they want to collect water using containers from the house, but regarding containers from the yard, no change is necessary, because all yards are one domain as was explained above in chapter 372 (Hagahot Maimoniot chapter 15) . ”