Cutting the rod in small diameter wasn’t a problem for me. Ideally it should be a square cut.
Ti oxidation layer protects it https://cdn.ymaws.com/titanium.org/resource/resmgr/2010_2014_papers/HouserRobert_2011.pdf
You may have two different sensors if you want to be able to do “water detection” v “conductance”
Also depends on power consideration, and how long it should be left between maintenance visits.
Conductance – ionic transport in water – is much studied and seems to get tricky pretty fast, but maybe it doesn’t have to be that accurate.
Stream water gets a lot of particles in it as flow is reduced.
Mechanical considerations is always a challenge. If you are looking at an array of sensors, – I wonder what distance between them. If they are going to be linearly distributed, then perhaps a PVC (1″? 3/4″) pipe could be a mechanical holder and wire conduit. Electrodes could be inserted in joining sections. That would make it accurate for intra-electrode spacing. There could be some challenges assembly two titanium electrodes with wires, inside a 1″ PVC pipe joiner, but seems doable.
The array of temperature sensors also is something I’ve been thinking of. I like thermistors for their low thermal inertia – however both need to be sealed from direct water contact, and that needs to be both thermally conductive and prevent long term water migration.
Thermistor v DS18B20 is partly how many units are likely to be produced. The DS18B20 require the individual ID numbers to be characterized in the array, thermistors require an analog multiplexing scheme. Both are dependent on how far from a datalogger.