I haven't nailed down the location of the pool but I don't think its a planet. I have interpreted it as simply "the area beneath the horizon" thus far. However, it may be the pool is more significant. I will turn to pp 182-183 of Lull and Belmonte where he speaks of a Mesopotamian origin
Lull and Belmonte
Among them, we can cite KU, the Fish, identical to our Piscis Austrinus (PsA), ASH-IKU, the Field, in the square of Pegasus, UGA, the Raven (Corvus) and MUSH, Hydra, normally related to the Lion and, finally, MA.GUR, the Bark (possibly Corona Australis), associated with the archer Pabilsag. Surprisingly, or perhaps not, all these can be easily identified in the Dendara Zodiac with a fish at the bottom of the water-pouring Hapi, an inundated field in the middle of Pisces, a bird and a snake close to Leo and a small bark (much larger in other representations)
Therefore, I would provisionally say the pool may be the field inundated with water, which is represented in the Dendera zodiac by the symbol a field with wavy water lines, essentially the Gardiner n39 hieroglyph. Referring to Wikipedia:
The description of n39 is a garden pool full of water. In the following Wikipedia article:
Under the notes for n39, it lists the following uses for n39:
Wikipedia n39 notes
1. block of stone, (or brick, etc.)
2. Ideogram or det. in inr for "stone", "block", etc.
3. Det. in types of building stones or types of minerals;
4A. Det. for dbt-(ḏbt), "brick"; 4B. dby-t-plinth, pedestal
4A is interesting because in the story of the birth of the children, following birth, each child is "placed on a couch of brick. Then Meskhenet approach him.." Meskhenet is a goddess of birth who is represented with a brick.
With the above, a better argument might be that the Pool is the Square of Pegasus. The crocodile is taking the commoner towards the Square of Pegasus ie into the Duat / below the horizon. The Square may also functionally overloaded into the couch the babies are metaphorically placed upon.