Water level Detection using Ultrasonic Sensor and arduino
A friend of mine worked on a liquid level monitoring project, and after many iterations finally abandoned the kinds of sensors you're talking about. Ultimately he just pointed a cheap USB webcam at the thing and used OpenCV and the edge detection algorithm to find the level of the water.  Then it's just a matter of doing some math using the dimensions of the container, to figure out how full it is.  

If you are using a clear container and if you have the option of using a Raspberry Pi (or a PC, whatever) that can run OpenCV, you might find that option worth exploring.