I was thinking of 1-wire, see www.hobbyboards.com to get the data from the sensor to the PC. For the sensor I can quickly think of 3 ideas, build a float/lever thing like a jumbo gas tank sensor, a float with a magnet in a PVC tube with magnetically operated (reed) switches at intervals, a float in a pvc tube with an ultrasonic distance sensor, or maybe you could get a good enough ping back directly from the water, (there are posts on here about an ultrasonic snow depth sensor, this could be very similar). What is the expected water level change?