I knew it was a mute point but interesting in knowing that the 555 will trigger when Vcc is removed from pin 2 without a pull down resistor. Apparently the internal resistance of the input is sufficient. Verified using a LM555 in my test circuit.

 

If no one has mentioned this:
sourcing map Stainless Steel Float Switch for Water Pump Tank garden pond Liquid Water Level Sensor M11 5.31" Length : Amazon.co.uk: DIY & Tools
You could make one of these.
Good luck.............

 

If no one has mentioned this:
sourcing map Stainless Steel Float Switch for Water Pump Tank garden pond Liquid Water Level Sensor M11 5.31" Length : Amazon.co.uk: DIY & Tools
You could make one of these.
Good luck.............

I have used those from Amazon, they actually work fine.

Ron

 

National Semiconductor used to make the LM1830 level sensing chip, which put AC on the probes to eliminate the risk of plating. The chip is out of production, but there might be a way to achieve the same results using a microcontroller.