The water analogy is rather basic and easily falls down as electrons are not water, which obeys the laws of hydrodynamics whereas electrons do not. Water also takes time to flow, electrons move unbelievably quickly.
I well remember an old lady going into a shop for a replacement light bulb. The...
Old Jim is entirely correct. The output of the normal 555 never goes to the supply rails. That introduces errors which will not only result in asymmetry of the output waveform but will make the timings both supply voltage and load dependent. The CMOS version may be better in this respect...
Sorry to be so late to this particular discussion but I only just saw it whilst searching for 555 circuits.
I think it worthwhile adding my contribution as no-one has suggested my solution and someone in the future may find it handy.
The reason it's usually impossible to get a 1:1 mark:space...