Here are relevant references from the Books where Water Carriers are mentioned.
While this group is not specifically titled a Caste, I include it here for reference.
I make no pronouncements on these matters, but report them as I find them.
Arrive at your own conclusions.
I wish you well,
"Water! Water!" called the man.
"Water," I said.
He came to me, bent over, tattered, swarthy, grinning up at me, the verrskin bag over his shoulder, the brass cups, a dozen of them, attached to shoulder straps and his belt, rattling and clinking. His shoulder on the left was damp from the bag. There were sweat marks on his torn shirt, under the straps. One of the brass cups he unhooked from his belt. Without removing the bag from his shoulder, he filled the cup. He wore a head scarf, the wrapped turban, wound about his head. It was of rep-cloth. It protects the head from the sun; its folds allow heat and perspiration to escape, evaporating, and, of course, air to enter and circulate. Among lower-class males, too, it provides a soft cushion, on which boxes, and other burdens, may be conveniently carried on the head, steadied by the right hand. The water flowed into the cup through a tiny vent-and-spigot device, which wastes little water, by reducing spillage, which was tied in and waxed into a hole left in the front left foreleg of the verr skin; The skins are carefully stripped and any rents in the skin are sewed up, the seams coated with wax. When the whole skin is thoroughly cleaned of filth and hair, straps are fastened to it, so that it may be conveniently carried on the shoulder, or over the back, the same straps serving, with adjustment, for either mode of support. The cup was dirty.
I took the water and gave the man a copper tarsk.
. . .
I finished the cup of water and handed the cup back to the water carrier. He bowed, grinning, the bag, swollen and bulging, damp on his shoulder, and, hooking the cup on his belt, backed away. "Water!" he called. "Water!"
Tribesmen of Gor Book 10 Pages 36 - 38
I saw the water carrier, too, with the brass cups. It struck me suddenly strange that he should be in the area of the bazaar, which, in the lower area in the city, is in the vicinity of the wells. Surely few would care to purchase water where it lay free at hand. He descended the steps and submerged his bag, grinning at me, remembering me from earlier in the day. I smiled at him, turning away. He was a simple, poor fellow, harmless, servile, slight. I felt myself a fool. Of course he would have to come to the bazaar area.
Tribesmen of Gor Book 10 Page 57
On the way to the street of weapon makers I again passed the water carrier. His bag on his shoulder was now, again, damp, dark, bulging.
"Tal, Master," said he to me.
"Tal." said I to him.
Tribesmen of Gor Book 10 Page 59
Looking up, I saw, hurrying toward us, carrying a torch, the small water carrier I had encountered several times. He looked up at me. "Did you see?" he asked. His face was white. "It was horrible," he said. He trembled.
"I saw," I said.
I pointed to the two men in the street. "Do you know these men?" I asked.
He peered at them closely. "No," he said. "They are strangers in Tor."
"Is it not late to carry water?" I asked him.
"I am not carrying water, Master," he said.
"How is it that you are in this district," I asked.
"I live but a short way from here," he said. Then he left, bowing, carrying the torch.
I looked at the man to whom I had spoken earlier. "Does he live near here?" I asked.
"No," said the man, "he lives by the east gate, near the shearing pens for verr."
"Do you know him?" I asked.
"He is well known in Tor," said the man.
"And who is he?" I asked.
"The water carrier Abdul," said the man.
"My thanks, Citizen," said I.
Tribesmen of Gor Book 10 Page 66