Have you tried snugging the screw down a bit?
Typically these are little more than a screw stud and a rubber washer/seal that is expanded by tightening the screw. I haven't replaced one myself, but it sure looks like it would be a very straightforward job (assuming, of course, the tank is emptied below the level of the sensor probe in question). If the tank is cracked or something where the probe goes in, that's a somewhat different matter.
