The Sharp IR distance sensors IIRC are meant for small distances less than a couple of feet. I had tried to work with one of these sensors and found the range to be severely limited. I found success with the PING from parallax and subsequently rolled my own PINGer using some locally available Ultrasonic sensors http://robokits.co.in/shop/index.php...roducts_id=100 I didn't get too great a range, but, straight line performance was better than 5 feet.

Regarding the doubts about sensor reliability, you do get the sealed sensors that are meant for liquid measurement applications. The site which will give you a good handle is Maxbotix. This link shows a sensor meant for such http://www.maxbotix.com/MB7067__XL-MaxSonar-WRC1.html