I've used these fluid level sensors before. They're optical, there is no electrical or mechanical interaction with the water. The LED and photodiode detect a change in refraction out of the sensing cone.
http://www.fluidswitch.com/pages/Optical-Liquid-Level-Sensors.htm
If you know anything about optics, you know that they work due to the index of refraction, which involves changes in material. Water to air, or air to glass, or glass to different types of glass...they'll all change the angle of light a different way. Also consider prisms; due to the index of refraction, light hitting a surface at a certain angle on an internal glass-air interface will reflect.
The angle of the cones on these fluid sensors appears to be close to the limit of refraction in air. Once water or some other fluid contacts the surface of the plastic, the light will probably no longer reflect off both walls of the cone and return to the photodiode.
You could either find some of these sensors or do some experiments and figure out