Actually , Junks were divided up into WT compartments - well , not ones that you could put much of a pressure in , but then in the ordinary junks ( not the extreme , possibly make-believe ones that are the subject of the thread ) being a flat-bottomed and fairly shallow draught design , you wouldnt have to have something totally watertight , just enough so that you could control flooding (hopefully) enough to bale her out , and to prevent free surface effect from rolling her over .
Pretty seaworthy things too . Someone sailed one back here to Falmouth from Hong Kong about 1900 or so , and that Irish navigator who recreates pre-historical ships , Tim Severin , tried to build a junk the old way and sail it on the northern circle sailing route east to the states across the Pacific . The traditional hull treatment was supposed to be stuff called "Tung Oil" inside and out , but there was something wrong there for they only got about halfway and the shipworms had chomped her full of holes , so they had to be lifted off . Shame .
Interesting rig , too , very much less tippy than those bermudan rigged things ( like my small 20 foot toy ) , and dead easy to sail apparently , once you get used to the apparent cats cradle of strings , although I've not had a go .
Bit of a dog going to windward though , but then it is a fat trading hull .
A guy called Blondie Hasler down in the west country used to be a complete fan of the rig about 30/40 years ago for the single-handed Trans-Atlantic annual race , he had a whole series of junk rigged yachts , all called "Jester"
As regards extreme size wooden ships didn't the mad roman emporer , Caligula , waste a boggling sum of money building two whoppers to go on a lake in Italy --Lake Nemo ? or something - anyway, they were supposed to be big enough to have a temple on one and his palace on the other , and when he'd got carved up for his sins , the things were abandoned , sank , and they've recently dug up some of the remains , and found they were not only much bigger than the old rumours and accounts , but also built with a sort of innovative grid of bulkheads inside , like a crate for wine bottles ( yes , that metaphor would occur to you , hammy , wouldn't it !
" Relax ! No-one else is going to be fool enough to be sailing about in this fog ."