Slightly off-topic, but since were talking level sensors; I thought I'd throw this in:
http://www.maxbotix.com/
For less that $30 you get a 1" x 1" ultrasonic sensor that outputs distance from 6-254 inches with 1 inch resolution in PWM, analog voltage, or 9600 baud serial.
It isn't water proof/resistant (yet, a more resistant version is coming), so you have to take precautions to keep it dry. I use short sections of PVC pipe and open-cell foam to protect it. I'm using them to measure the draft of coal barges off the coast of Colombia and they work great.
Terry