Hi,
I realise you want it as simple as possible..I'm just unsure as to why you are using a 555 timer.
From what I've read, you simply want a small circuit that makes the output 'high' whenever the two wires are imersed in water...correct? Now, even though 'pure' water has a pretty high resistance., no connection has virtually infinate resistance. So all you need is something that detects this change in resistance. I would say, a comparator or a transistor amplifier.
heres some links that may help:
**broken link removed**
**broken link removed** - uses comparators.
**broken link removed** - you only need one probe.
I also believe logic inverters are sensitive enough to detect the smaall current that flows through the water...I mean, if you leave their inputs unconnected the output goes crazy. So a pullup/down resistor should be used.
Hope this helps,
Blueteeth