三徒共溺 Three Contributions
那獃子揭衣服，忽喇喇，就似呂梁洪倒下板來，沙沙的溺了一砂盆。沙和尚卻也撒了半缸。依舊整衣端坐在上，道：「小仙領聖水。」那呆子揭衣服，忽喇喇，就似吕梁洪倒下板来，沙沙的溺了一砂盆。沙和尚却也撒了半缸。依旧整衣端坐在上，道：「小仙领圣水。」
逐句解 Line by line
That idiot hoisted his clothes and — with a great rushing roar, like the Lüliang cataract crashing down its weir — pissed out a whole sand-basin's worth.
那獃子: 那 = nà; 子 = zi. The joke is the simile. 呂梁洪 is the Lüliang cataract, a stretch of rapids proverbial for sheer force -- 莊子 tells of a swimmer who survives it by going with the water. The novel reaches for that famous image and applies it to Bajie urinating into a basin, which is exactly the kind of mock-grandeur the whole scene runs on. 倒下: 倒 = dǎo, to crash down. 就似: 似 = sì.
Friar Sand too let go half a vat.
卻也 "he too, for his part." 撒了半缸: 了 = le; a 缸 is the big vat, so half of one is a considerable quantity.
Then, as before, they straightened their robes and sat up on high, and said, “Little immortals, receive your holy water.”
依舊 "as before." 端坐在上 "sitting up straight in the high seat" -- back into the idols' poses. 領聖水 "receive the holy water," delivered in the flattest priestly tone available.
Each disciple adds his measure, and the novel dignifies Bajie's with a comparison to one of the most famous rapids in Chinese letters. That gap -- epic simile, squalid act -- is the engine of the passage.
Then all three straighten their robes, resume the statues' posture, and announce 領聖水 in the voice of a god conferring a favour. The whiplash between what they have just done and how they announce it is the joke in its purest form, and the novel holds the pose for exactly one line before moving on.