So of course I then asked the internet if such a wonderful steampunkesque* creation existed. I couldn't think how it would work right off, but I leave that to other creative minds to solve. The answer? Think Geek sell a water-powered clock that's just six inches tall, small enough to be portable. But there's even better to come: there will be water-powered mobile phones available (at least in Korea) by 2010. And a mobile is (among other things) a kind of pocket watch.
* Or, quite literally, clockpunkesque, but I wasn't mentally committing to how the waterworks inside were going to function.