I use a piece of PCB with fingers, like this one: http://www.circuitstew.com/pcbs/water_alarm.htmlAn Arduino is certainly overkill as you need only about four parts to make a very reliable leak detector. Google "water leak alarm circuit" for thousands of possibilities.
A float switch is more reliable than wires
I've used this ic before. It works great.
A float switch is more reliable than wires. The DC current through the wires will corrode them relatively quickly. You may need to build a dam around the pump to get a deep enough puddle for the float switch to float.Sump pumps and bilge pumps do the opposite. (Turn on when water detected.) Perhaps their float switches are available separately?
For the sensor, two bare wires placed in different holes in corrugated cardboard.
The more appropriate circuit . . .