That's not really correct. Around about the close of the Qianlong Emperor's reign, China was possibly the most prosperous it had been, at least since the golden age of the Song dynasty. There was something on here not too long ago about the industrial revolution in Britain that went into this in some detail[1] - standards of living were a fair amount higher, per-capita, than in Western Europe at the same time.
There is very little that China would, or ever did, want to import from Europe. This was a major historical problem for Western traders in the China market - outside of bullion, and furs, to some extent, they had a real hard time finding products that they could successfully trade.
It's not as though China was bereft of the capabilities of fielding a large, powerful fleet - the Zheng He expeditions in the Ming dynasty were quite impressive, even if the accounts are somewhat exaggerated, and some of the biggest naval battles in history have been fought on Chinese lakes and rivers. But historically, the government was always facing serious threats from the interior of Asia, whereas coastal threats were generally little more than pirates. This has some knock-on effects - less incentive to study astronomy and geometry, when you aren't using them to sail ships around the world, different approaches to fortification - the Great Wall was more of a Ming thing, whereas the Qing, as an outsider foreign dynasty, tended towards assimilation of Mongols and other dangerous nomads into the ruling elite and the banners instead.
[1] http://www.csun.edu/~jaa7021/hist531/Goldstone%20-%20Efflore...