drifting ice floes

The two watchmen trailed through the slush and muck to the Water Gate, by which the river Ankh had the good fortune to enter the city. The water was invisible in the dark, but the occasional ghostly shape of an ice floe drifted past below the parapet.
“Hang on,” said Nobby, as they laid hands on the windlass of the portcullis. “There’s someone down there.”
“In the river?” said Colon.
He listened. There was the creak of an oar, far below.
Sergeant Colon cupped his hands around his mouth and issued the traditional policeman’s cry of challenge.
“Oi! You!”
For a moment there was no sound but the wind and the gurgling of the water. Then a voice said: “Yes?”
“Are you invading the city, or what?”
There was another pause. Then:
“What?”
“What what?” said Colon, raising the stakes.
“What were the other options?”
“Don’t mess me about… are you, down there in the boat, invading this city?”
“No.”
“Fair enough,” said Colon, who on a night like this would happily take someone’s word for it. “Get a move on, then, ‘cos we’re going to drop the gate.”
After a while the splash of the oars resumed, and disappeared downriver.
“You reckon that was enough, just askin’ ‘em?” said Nobby.
“Well, they ought to know,” said Colon.
“Yeah, but–”
“It was a tiny little rowin’ boat, Nobby. Of course, if you want to go all the way down to them nice icy steps on the jetty–”
“No, Sarge.”
“Then let’s get back to the Watch House, all right?”

– guardians of the city | Terry Pratchett, The Truth