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.
Deictic 那獃子: 那 = nà; suffixal 子 = zi. 呂梁洪 the Lüliang rapids, a cataract proverbial for its force (cf. 莊子). 倒下: 倒 = dǎo, “to crash/topple down.” 就似: 似 = sì.
Friar Sand too let go half a vat.
卻也 “also, for his part.” 撒了半缸: 了 = le; 缸 a large earthen vat.
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 “to sit up straight in the high seat” — resuming the idols' poses. 領聖水 “receive the holy water” — the mock-solemn summons.
Each disciple adds his measure — the mock-heroic simile of the Lüliang cataract dignifying Bajie's contribution to absurd proportions — and then all three resume their statuesque calm to “bestow” the water. The tonal whiplash between the crude act and the priestly diction (小仙領聖水) is the engine of the whole scene.