Water level: BIG capacitive level sensor with steel bars?
Nice project. Any conductor, even relatively weak ones, will do because the capacitance will be in series with either &lt;1 ohm or 100 ohms+ and the resistance won't make a deal of difference. Water temperature, contaminant levels, salinity and eels may all conspire to give you a false reading though but good luck and you should get fairly decent reults. Do you plan to use the "capacitor" in an oscillator circuit?

One more thought - just in case the water physically touches the probes avoid any DC levels in case these give you "other" effects like killing an endangered species of blanket weed.