Water in what sense? Flood? Humidity?

Plenty of humidity sensors.

I have a rain sensor which has an ADC converter built in so when it's wet/dry it just outputs high/low which is easily read on the Pi GPIO.

Search eBay for Arduino rain or moisture sensors and see if the idea of them makes sense for you.

Water as in flood.  We've had our A/C drain plug and spit water on the floor more than once.  I did do a quick search after I posted and it seems most of them are used for moisture in soil.

Well who would believe that someone already did most of it?  I figured that was the case but was too lazy to look until now!

There you go then!

But I believe the soil moisture sensors can be used to detect water as a wet/dry variable rather than percentage of moisture.

That may be what I go with, I'll try making this one first.  The ones you buy just look better.

That's what I've got

http://pages.ebay.com/link/?nav=item.view&id=281371131769&alt=web

You could just remove the sensor plate, since its basically just a load of metal tracks that get bridged by water to make a circuit, and run two wires to the floor where the water will be.

Hmm, so if any of that gets wet it registers right?  That might work just fine.

Look what this guy made!  Our farm isn't that big but this would still be pretty useful!

Yeah if you wet your finger and touch the plate it registers. Or put a drop of water on it.

Well you encouraged me to actually setup the rain sensor.

Wow, lots of data!  Sweet!

Turns out my cheap rain sensor corroded in the rain so it has stuck on! Oh the irainy. (sorry)

Irainy oxidey