I use these sensors to monitor water level in a large HDPE storage tank for domestic water. They have worked well.
See my post here for some additional information:
http://www.robotshop.com/forum/post-p128300#p128300
You may need to include code to debounce a sensor if the water level in the tank partially covers the sensor area. This problem would occur as the water level rises and falls and just happens to stop about the middle of the sensing area.