I was looking for something similar a few weeks back, except my requirement was more complex as I wanted to detect 10 different levels of water using a simple 2 wire probe.
Anyway, while I cant pretend to have found the perfect solution, Arrati's suggestion in that thread (http://www.picbasic.co.uk/forum/showthread.php?t=11468) came pretty close.
So, my vote would be stainless steel nut-bolts to make the actual contact, with a good resin epoxy sealant around the wire connected to the bolt. This coupled with a software arrangement to drive the sensor with a duty cycle of perhaps 1:10, or even 1:100 would give you years of maintenance-free use.
Incidentally, I used the adc of the pic, rather than the comparator, although in your case the comparator (or even a digital input) should be fine.
Regards,
Anand Dhuru
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