水勢極熟 The Catfish In The Pool
獃子不容說，丟了釘鈀，脫了皂錦直裰，撲的跳下水來。那怪心中煩惱，一齊上前要打。不知八戒水勢極熟，到水裡搖身一變，變做一個鮎魚精。呆子不容说，丢了钉钯，脱了皂锦直裰，扑的跳下水来。那怪心中烦恼，一齐上前要打。不知八戒水势极熟，到水里摇身一变，变做一个鲇鱼精。
逐句解 Line by line
The blockhead would hear no more of it. He threw down the muck-rake, pulled off his black brocade habit, and jumped into the water with a splash.
直裰 is the long habit a monk wears — zhí duō, and the second character is rare enough to be worth having. Two 了 in twenty-one characters, both the light le marking things done and got out of the way.
The monsters were furious and came at him together to strike.
心中煩惱 is vexation, distress in the heart — twelve characters, and it is the reasonable reaction of seven people whose bath has been invaded.
What they did not know was that Eight Rules is thoroughly at home in water. Once in, he shook himself and turned into a catfish spirit.
水勢極熟 — completely practised in the ways of water. He was Marshal of the Heavenly Reeds, in charge of the celestial river, before he was a pig. 鮎魚 is a catfish: no scales, and impossible to hold.
He throws off his habit and jumps in, and the fight is on. Seven against one, in chest-deep water, with nobody dressed.
The catfish is a good choice and the book tells you why in four characters: he is thoroughly at home in water. Before he was a pig he commanded the celestial river, and that history is still doing work seventy chapters later.
A catfish is specifically the fish you cannot hold. No scales, all mucus, built to slip out of a hand. He has picked the form for the situation, which is more thought than he usually gives anything.