Historically, a caïque was a boat of 5–6 meters (16–20 ft) in length, and 1 meter (3 ft) in width, used mainly for transportation. It had a shape similar to that of a skate. Both ends of it were in such a form that it could be rowed in either direction with equal ease. The sides consisted of two long embellished boards. There were also lateral boards supporting the sides against water pressure.
There were also imperial caïques used by the Ottoman sultans and his suite for ceremonial and daily excursion purposes. The size and grandeur of this type, adorned with imperial armorials and floral scrolls, was a reflection of his royal power. Another feature of imperial caiques was the deck pavilion, a partition or a small kiosk for the Sultan. By reason of this feature they were also called the pavilion caïques.
Picture of imperial caïque to be found here. Three feet wide, though? That's worse than a canoe. I see why he said that if you stand up in a caïque you have three seconds before it tips.
In other news, Mieville's imagination makes me feel grubby no matter what he's writing. When he's writing worshippers of the giant squid it gets worse. And he still has this obsession with turning people into machines, if not actually Thomas the Tank Engine.