You misunderstood (or i didnt get my point across). Anyway, the problem is not the metal in water, it's the magnetic field.
The initial testing with these types of sensors, magnetic displacement in flow measurement (to be more precise, but it can be interpolated to here), using a magnetic sensor, resulted in precipitation of ions on the surface of the magnet (which is isolated, but doesnt matter as long as the field exists), distorting the field, and giving inaccurate readings, or even reducing the field considerably , with some extra EMI , making the sensor useless.
However, as i said, distilled water with a low ions count, a magnet with a light field, and those reed switches , in theory, they WILL work. The whole issue is the remaining ions in the water, and the long term effects on the magnet . Using that setup and having the whole thing stop working properly in two weeks isnt what we want
.
But "gopher it" , and give us some feedback
. I'm having trouble locating some reed switches here for some testing. Damnit.