Ok, I see I missed a few details:
This would be for both basement flood due to rain (so dirty water) and also flood from pipe leak under sink (tap water). It doesn't happen often, so the leads are usually dry. I'm assuming tap water, once it washes up on the floor for a bit, gathers enough salts to be conductive? But I'll probably have to do some experimenting to see.
The bread-board Arduino is battery powered. I was hoping the input would go high (or low) and act as an interrupt to wake up from sleep. So my preference is for a digital input. Then I'd program in the nRF24L01 to send out a couple of pings to a gateway, which will then send an email alert.
Thanks for the clarification on "ground". I though that if there is a metal pipe near the water, the 5V would ground...but that's not really true as the negative end of the battery isn't connected to earth? Things in the house might ground out because the house shares the same ground plane as the source of the 120V power? Perhaps that's not quite right, not sure. But the gist is if I put the positive end of a battery terminal to a metal pipe in the house, it's not going to short the battery, right?