Can you be more specific. Are you measuring the depth of the water? PH or chemical make-up? Temperature? or all of the above? Ultimately that would help decide the type of meter you would be looking at purchasing then a search for code that has already been made for that meter and the Pi.
http://www.raspians.com - always looking for content feel free to ask to have it posted. Or sign up and message me to become a contributor to the site. Raspians is not affiliated with the Raspberry Pi Foundation. (RPi's + You = Raspians)