金箍探水 The Gold-Hooped Staff Sounds the Water
好大聖，把金箍棒拿出來，兩頭一扯，叫：「長！」足有七八丈長。教：「八戒，你抱著一頭兒，把你放下井去。」八戒道：「哥呀，放便放下去，若到水邊，就住了罷。」行者道：「我曉得。」那獃子抱著鐵棒，被行者輕輕提將起來，將他放下去，不多時，放至水邊。八戒道：「到水了。」行者聽見他說，卻將棒往下一按。那獃子撲通的一個沒頭蹲，丟了鐵棒，便就負水，口裡哺哺的嚷道：「這天殺的，我說到水莫放，他卻就把我一按。」好大圣，把金箍棒拿出来，两头一扯，叫：「长！」足有七八丈长。教：「八戒，你抱着一头儿，把你放下井去。」八戒道：「哥呀，放便放下去，若到水边，就住了罢。」行者道：「我晓得。」那呆子抱着铁棒，被行者轻轻提将起来，将他放下去，不多时，放至水边。八戒道：「到水了。」行者听见他说，却将棒往下一按。那呆子扑通的一个没头蹲，丢了铁棒，便就负水，口里哺哺的嚷道：「这天杀的，我说到水莫放，他却就把我一按。」
逐句解 Line by line
The fine Great Sage took out his gold-hooped staff, gave it a pull at both ends, and cried: 'Grow!'
金箍棒, the gold-hooped cudgel — banded at both ends, and famously obedient about its own size. 兩頭一扯: a pull at either end. 叫「長！」: he tells it to grow (長 = zhǎng, 'to grow,' not cháng, 'long').
It grew to a full seven or eight zhàng in length.
七八丈長: a full seven or eight zhàng in length (丈 = zhàng, left in Chinese as English has no borrowed name for it; 長 here is cháng, 'length' — the same character that meant 'grow' one line ago, now doing its other job). A zhàng is ten chǐ, so the staff is a serious length of sounding-line.
He said: 'Bajie, hold on to one end, and I'll let you down into the well.'
抱著一頭兒: hold on to one end (著 = zhe; 兒 the light ending on 頭). 教 here is jiào, 'he told him.'
Bajie said: 'Brother — lower away, but when I reach the water's edge, stop right there.'
The sensible instruction, given clearly, in advance, and about to be ignored. 就住了罷: 'stop right there' (罷 = ba).
The Pilgrim said: 'I understand.'
曉得: 'I understand' (得 = dé). Two words, and the reader can already hear what is coming. 行 = xíng.
The blockhead clung to the iron staff, and the Pilgrim lifted him lightly up, then let him down; in no long while he was lowered to the water's edge.
提將起來…放下去: lifted up, then lowered away (將 = jiāng, twice). 那 = nà; 著 = zhe.
Bajie said: 'I've reached the water.'
The signal, duly given.
Hearing him say so, the Pilgrim gave the staff a push straight down.
And the shove. 卻將棒往下一按: gave the staff one push straight down (將 = jiāng). 行 = xíng.
With a splash the blockhead went under headfirst, let go the iron staff, and began treading water, sputtering out of his mouth: 'That murdering wretch! I told him not to lower me once I hit the water — and he pushes me straight down!'
撲通 is the splash, a sound-word as plain in Chinese as it is in English. 沒頭蹲: a head-first ducking (沒 read mò, 'to submerge,' not méi, 'not have'). 負水: to tread water. 哺哺 is the sputtering of someone talking with a mouthful of well. 這天殺的, 'this heaven-slain wretch,' is a good round curse and exactly what you would expect from a wet pig. 那 = nà.
Wukong's 'trick' is his own staff, stretched into a lowering-line, and the descent turns into the pair's characteristic byplay. Bajie, going down carefully, gives the sensible instruction to stop at the water — which Wukong, the instant he hears the pig reach it, disregards entirely, shoving him under head-first with a splash.
The dunking is pure slapstick, and Bajie's response is the comedy of the put-upon companion: treading water and cursing his brother for the precise betrayal he had tried to forestall. The scene trades on the reader's affection for both of them. Wukong's mischief is cruel only in the way farce is cruel, and Bajie's outrage is more grumble than grievance.
Underneath, the plot has advanced exactly one necessary step. Bajie is now in the well, treading its water, which is where he must be to recover the king. The false treasure, the extending staff and the sudden shove have all served one end: getting a reluctant pig to the bottom of a shaft.