Collection of circuits:
A is the proper 555 circuit with plenty of noise immunity and hysteresis.
B is the same thing implemented with CMOS gates. Good noise immunity, but the exact amount of hysteresis is quite variable.
C is the same as the 555 circuit implemented with a rail to rail op-amp or a comparator with Push-pull output.
D is the same as the 555 circuit implemented with a standard LM339/LM393 comparator with open collector output.
E is Mr. Schmitt’s original trigger Implemented with MOSFTEs to give a high input impedance. With no water the first transistor is on, and the voltage across the 75Ω resistor is about 130mV. When water is present, it starts to turn the first MOSFET off, which turns the second on. The relay current is about 20mA, which drops 1.5V across the 75Ω resistor, feedback is positive and the circuit changes state abruptly. It the needs the voltage at the input to change by 1.5V to switch it off again.

Back in the mists of time, National Semiconductor used to make the LM1830 fluid level detector IC. That used AC to avoid plating/dissolving the probes. It can also be done by sampling briefly at a low frequency, say sampling for 1ms every second.

The carbon anodes of zinc- carbon D cells make good probes as well!

 

A is the proper 555 circuit with plenty of noise immunity and hysteresis.

Lets just discuss A.
Well first off you drew the circuit wrong. The output is normally low when the sensor is not in contact with the water.
That's OK it's an honest mistake. It works good uses one less component and a 1uf cap will give some hysteresis depending on the sensor resistivity.
However another reason I chose my version is because the TLC555 will sink current better then source. The 5 volt relay with a 68 ohm coil will not operate in the source mode, output drops to 1 volt. I know you can add a transistor to drive the relay but I preferred not.
SG

 

But you've got a changeover relay - you can use the normally closed contacts.

You're still short of drive current: 5V/68 ohms is 73mA and the TS555 will sink 50mA.
Last time you drew it, it was a TLC555 and that will sink 100mA.
(Just to complete the set the ICM7555 only manages 20mA)
But if you need more sink current, join pin 3 to pin 7.

 

Last time you drew it, it was a TLC555 and that will sink 100mA.
[/QUOTE
grammatical error. Corrected above

But you've got a changeover relay - you can use the normally closed contacts.

Yeah but you don't want the coil energized when in the standby mode that's just wasting power.
Final note let's just agree to disagree OK?
SG

 

Various varieties of 555 are not always quite so interchangeable as one would like to think!