Hi Paul,
Your thinking aligns with mine. I resist the idea of sinking probes (particularly dc fed) into water tanks because of the electrolysis effect for one thing, although that can be reduced using ac or even reverse polarising dc every alternate sampling. In situations where I've used wet probes, I've used that reversing the polarity approach along with only powering the probes during measurements.
That way, the life of the probes can be extended quite significantly. Despite that, electrolysis also causes changes in the water chemistry, which I only understand superficially, but which I'd rather avoid.
I use floats in my water tanks for that reason. Mostly I care if there's risk of overflow or risk of empty tank, and I am less concerned about reading intermediate levels, but then I live in a country that never goes long without rainfall! In addition, I built an ATMEGA based weather station with a rain gauge so I always have a clue how much rainfall we're getting.
With respect to the model the OP has chosen, the probes do give a clear distinction between covered and uncovered, so it can be interpreted as on or off (although it is not exactly that). The difficulty is always that the actual readings will change over time as the probes are degraded by electrolysis. That's my ongoing concern and why I also avoid using wet probes too.
Hey, interesting discussion and thanks for your comment. Good luck with future projects and best wishes to you.
Steve